The "three fingers below" technique is one that we're encouraged to start beginners with. It's a useful technique when shooting without a sight, since it brings the arrow closer to the eye.
The Mediterranean hold -- one finger above, two below -- is what we use when target shooting, since it's the better technique when using a sight.
There's some controversy about which technique should be taught to beginners -- whether there's any point teaching the "three below" hold and then dropping it and teaching the Mediterranean hold. See this thread, for example:
http://www.archery-interchange.com/f...?t=1681&page=4
(starting with Rik's post, 4 down).