i used to be all for a three man defences, with two dm's.but now after playing the formation for colleges, i have come to the conclusion that unless we have AMAZING wing back/wingers, and centre back with bomb loads of pace and reading ability it doesnt work.
four man defenc , imo is the most flexible for me....two flying fullbacks that can aid the attack when needed...
(its the arsenal formation, it works beautifully, absoloutly perfect.)