3-5-2 in and of itself isn't the problem. The formation can work well if a side has the proper players on the wing, and some pace in attack.
The problem with our current 3-5-2 is we don't have the wingbacks, nor the fast, clinical forwards to really make it work. Llorente sucks in a 3-5-2. Lichtsteiner and Asa suck in a 3-5-2 offensively, so we end up with an ultra-defensive 3-5-2 that relies heavily on the midfield contributing goals, and a quick, technical forward like Tevez to create chances. We rarely look like conceding against weaker sides, but against better teams, chances eventually come and we inevitably concede even though we have 5 players at the back because our defenders are forced to push forward with a very high line to play keep away in the midfield area while waiting for a chance to send a ridiculously hopeful long ball over the top in hopes of someone getting on the end of it. This leaves us terribly vulnerable to quick counterattacks.
If we had pacy, offensive-minded wingbacks, and a Tevez--Falcao (perhaps Morata if he can get better at finishing) forward combo, our 3-5-2 could be quite good. We don't, so it's really time it is put to death for this squad as our management has shown no inclination to go out and remedy these problems.