Can I use a three-point saddle over a flat duct?

You can, but the crown of a three-point saddle is a point, so it clears the duct only at the centre and the pipe slopes down on either side of it. Over anything wider than about its own diameter the ends of the obstruction end up inside the slope. A four-point saddle puts a flat across the top and clears the full width.

Is a four-point saddle just two offsets?

Exactly that: an offset up, a straight length across, and an offset back down, all four bends on the arrow. Which is why its calculator uses the offset multiplier and the offset shrink, twice.

Why is the three-point saddle’s centre bend twice the outer angle?

Because the two outer bends between them have to undo what the centre bend did. The centre turns the pipe up by the centre angle; each outer turns it back by half, so the pipe leaves level. If the outers were not half, the far leg would not be parallel to the near one.

Which one shrinks more?

At the same depth and the same bend angle, neither: both have two sloped legs of that depth, so both eat 2 × depth × the shrink figure for the angle. What differs is the angle each is normally pulled at — a 45° three-point has 22.5° outers and eats about 0.40 × the depth, a four-point pulled at 30° about 0.54 × the depth. The figure each calculator prints is not the same quantity either: the three-point gives one offset’s worth, because that is the correction added to the centre mark, while the four-point gives the full two offsets, because that is the run the saddle eats.

Three-point vs four-point saddle

Use a three-point saddle over something round and narrow — a pipe or a cable — and a four-point saddle over anything flat or wide, like a duct or a beam. Both carry a run over something in its way and bring it back level on the other side. The three-point does it with a peak and the four-point with a flat, and that one difference decides everything: which obstruction each suits, how many marks, which references on the bender, and how much run each one eats.

The two shapes

Three-point: centre bend on the notch, outers on the arrow at half the angle. It clears a point.

Four-point: two offsets back to back with a flat between, all four bends on the arrow at one angle. It clears a width.

Which one, when

The obstruction is…	Use	Because
A pipe, a conduit, a cable, anything round and not wide	Three-point	The peak sits over the crown; the slopes either side clear the sides of a round thing naturally.
A duct, a beam, a box, a rack — anything with a flat top wider than your pipe	Four-point	The flat across the top clears the full width at one height; a peak would only clear the middle.
Something round but big — a large pipe or a drum	Four-point	Once the round thing is wider than a three-point's slopes can clear, treat it as a width.
Two things close together	Four-point over both	One flat over the pair is cleaner and pulls better than two saddles back to back.
A small round thing, and the run is short	Three-point, 60° centre	Three bends in the least length; the steeper centre keeps the marks close.

The rule of thumb: round and narrow, three; flat or wide, four. When in doubt, four — it is only one more bend, it is two offsets you already know how to do, and a flat clears a width that a peak cannot.

How the marks differ

Three-point

Three marks: C at the centre measurement plus the shrink; 1 and 2 the centre-to-outer distance either side of C.
Centre-to-outer = depth × the offset multiplier for the outer angle (22.5° for a 45 saddle: ×2.613).
Centre-mark correction = depth × the offset shrink for the outer angle, once — that is how far past the crown measurement C goes, not the run the saddle eats. The run it eats is twice that.
References: C on the notch, 1 and 2 on the arrow.
Order: centre first (45°), then the outers (22.5° each) the opposite way.
Calculator: three-point saddle.

Four-point

Four marks: 1 at the layout measurement; 2 the between-marks distance on; 3 the obstruction width past 2; 4 the between-marks distance past 3. The shrink does not move mark 1 — it comes off the far end.
Between marks = depth × the offset multiplier for the bend angle (30°: ×2; 45°: ×1.414).
Shrink = depth × the offset shrink for the angle, twice — two offsets. Add it to the overall length, not to mark 1.
References: all four on the arrow.
Order: 1 up, 2 back (roll a half turn between), then 3 and 4 the same as a mirror image, so the pipe ends level.
Calculator: four-point saddle.

One obstruction, both ways

A 4" (102 mm) pipe crosses the run and I want 1" (25 mm) of clearance over it, both of them lying on the same surface. My pipe has to lift its underside from the surface to 1" (25 mm) above the other pipe, so the rise is 5" (127 mm). My own pipe's thickness does not come into it: its centre already sits half a diameter up before the saddle, and it is still half a diameter up after, so that term cancels — adding it is the edge-to-edge mistake in disguise. Here it is as each saddle, then the same job on a metric site.

As a three-point saddle, 45° centre

Outer angle 22.5°, multiplier 2.613, shrink 0.199 × the depth. Centre to each outer: 5 × 2.613 = 13.07 → 13 1/16" (332 mm). The centre mark C goes 5 × 0.199 = 1" (25 mm) past the crown measurement. Three marks, the whole saddle 26 1/8" (664 mm) from mark 1 to mark 2, and it eats 2" (51 mm) of run — two sloped legs, 1" (25 mm) each. The peak clears the crown, and the slopes clear the sides of a round pipe easily.

As a four-point saddle at 30°

Multiplier 2, shrink 0.268 × the depth. Between marks: 5 × 2 = 10" (254 mm), each side. Flat across the top: the pipe plus a little, say 6" (152 mm). Shrink: 2 × 5 × 0.268 = 2.68 → 2 11/16" (68 mm). Four marks, 26" (660 mm) long all told, and 2 11/16" (68 mm) of run gone against the three-point's 2" (51 mm). The gap is 11/16" (17 mm), not 2" (51 mm) — so the case for the three-point on a small round pipe is one bend and one mark fewer, not a big saving in run.

The same job in metric

A 100 mm pipe with 25 mm of clearance: the rise is 125 mm. Three-point at 45°: centre to each outer 125 × 2.613 = 327 mm, C goes 125 × 0.199 = 25 mm past the crown, and it eats 50 mm of run. Four-point at 30°: 250 mm between marks each side, a 150 mm flat, and 2 × 125 × 0.268 = 67 mm of run eaten. No fractions to round — the multipliers do not care which tape you hold.

Now make it a 12" (305 mm) duct

Same 5" (127 mm) depth, but the thing is 12" (305 mm) wide with a flat top. A three-point saddle's peak would clear the middle of the duct and the pipe would be sloping down into the duct's edges 6" (152 mm) either side — at 22.5° the pipe drops 6 × tan 22.5° ≈ 2 1/2" (64 mm) over that distance, which is more than the 1" (25 mm) of clearance. So four-point: 10" (254 mm) between marks each side, a 13" (330 mm) flat across the top, and 2 11/16" (68 mm) of run eaten to add to the overall length. That is the whole decision in one number: the slope of a three-point saddle drops about 0.41 × the distance out from the crown at 22.5°, and if the obstruction is wider than your clearance can absorb, you need the flat.

When either would do

On a small round obstruction with room either side, both work and I pick by what is easier at the bender: the three-point is one mark fewer and one bend fewer, but it needs the notch and a 45 pulled true. The four-point is two offsets, which is the bend I can do without thinking, and it lays flat on the floor to check. A first-year should learn the four-point first for exactly that reason — it is nothing new — and the three-point once the offset is automatic.

Questions

Can I use a three-point saddle over a flat duct?: You can, but the crown of a three-point saddle is a point, so it clears the duct only at the centre and the pipe slopes down on either side of it. Over anything wider than about its own diameter the ends of the obstruction end up inside the slope. A four-point saddle puts a flat across the top and clears the full width.
Is a four-point saddle just two offsets?: Exactly that: an offset up, a straight length across, and an offset back down, all four bends on the arrow. Which is why its calculator uses the offset multiplier and the offset shrink, twice.
Why is the three-point saddle’s centre bend twice the outer angle?: Because the two outer bends between them have to undo what the centre bend did. The centre turns the pipe up by the centre angle; each outer turns it back by half, so the pipe leaves level. If the outers were not half, the far leg would not be parallel to the near one.
Which one shrinks more?: At the same depth and the same bend angle, neither: both have two sloped legs of that depth, so both eat 2 × depth × the shrink figure for the angle. What differs is the angle each is normally pulled at — a 45° three-point has 22.5° outers and eats about 0.40 × the depth, a four-point pulled at 30° about 0.54 × the depth. The figure each calculator prints is not the same quantity either: the three-point gives one offset’s worth, because that is the correction added to the centre mark, while the four-point gives the full two offsets, because that is the run the saddle eats.