I have several 3 legged ladders, tallest is 12' high. Also have several 4 legged ladders.
The main advantage to these ladders is they work well on uneven ground, whereas a 4 legged ladder is not very stable unless it is on a flat surface. The other advantages already mentioned like getting into a corner, etc. The disadvantages are they are wide in taller heights, don't stack very well on the truck, etc.
Now the older wood 4 legged ladders may not be OSHA approved, not sure. And I heard that Red Top ladder, one of my favorite wood 3 legged ladders can not get insurance anymore which may have something wiht being approved.
But Werner 3 legged ladders are OSHA approved, see link below.
http://www.wernerladder.com/news/article.php?NewsID=4