The iphone doesn't currently use three points, but it's in the spec. (This was discussed in some detail quite awhile ago). Nothing uses three fingers yet because it's awkward, particularly on a small phone.
The trackpad used by most laptops (can't speak to apple's) seems to only be physically capable of spitting out, at best, rectangles. Go take a look at the technical specs or linux driver for synaptics touchpads.
Think about how it would have to work. In a normal touchpad, you press at x,y, and you are intersecting two lines (one running vertically at x, one running horizontally at y). At the edges of the pad, amplifiers detect this and send the x/y as the result.
Since your finger isn't a point, you are actually intersecting x1->x2 and y1->y2. So a finger intersects a range of x and a range of y. Again, at the edge of the pad, amplifiers detect this, and send off the appropriate ranges.
If you have two vertically stacked fingers, it increases the range of y, but the range of x stays the same. So it sends off an increased range of y, that software can interpret as a "double touch." And if it's only two fingers, it can sort of guess at where they are. But it's only a guess, not multitouch sensing in which each x,y can generate its own signal.