Federer was in all 4 slam finals in 2009. Both that he lost, he had on his racket. He won 2 of them.
It is far from ridiculous to say he could win 3 this year. It certainly isn't impossible. 4 isn't impossible either. It depends on his form and that of other players. A few matches at exhibition don't really mean much, come on. Exhibition is practically meaningless. Federer lost to Roddick in an exhibition in 2007 before winning the AO without losing a set. Meaningless.
It think it is possible that Federer wins 3 slams this year.
Imo, AO is 40%, FO 40% (assuming Nadal is in good form), Wimbledon is 70-80%, USO is 40%. This is my opinion (which is actually a little bit pessimistic, but very little is clear right now). You can assign your own probabilities, but the point is, no matter which way you look at it, suggesting he could win 3 slams isn't absurd. If you're assigning 0% probability to him winning any of the given slams, you're the one being absurd.