I like 3 at the back and think it cover for a load of our deficiencies. Martinez is getting a lot of criticism, and it is a potential weakness having him in a back two, but it's also not unworkable. There's also the fact that none of our CBs, other than Varane, are of great quality. Maguire in particular is slow, and Lindelof passive and weak the closer he gets to our goal, they'd each benefit from being RCB in a 3 with Varane covering. Plus, all of our weaker CBs are to an extent good with the ball at their feet. Being RCB or LCB in a back three allows them more scope to bring the ball forwards. It's not just about bolstering the defence.
Also there's the fact that our wide forwards are not great when they're out on the touchline, they're preference is to come more central. Wing backs pushing high and wide would free them up. Our centre mids would also be under less pressure with greater numbers for passing options, and split responsibility to try and get the ball between opposition lines. The only major weakness going for a back 3 with our present squad is RWB. And we need a RB anyway, so one who would be equally comfortable RB or RWB would give us plenty of options.
Another problem of our own making is attempting to play out from the back in a back four. Even teams playing with 2 CBs tend to build possession with a back 3, and ETH at Ajax did so too (earlier on with De Jong dropping from midfield to LCB, and later on Blind moving inside from LB). Against a decent pressing team, it's a big ask to build from the back in the manner we attempted the first couple games, and our players individually don't excel in doing so. Building possession from the back 3 would help us immensely. Although, this could be done without playing 3 CBs, if a full back or DM moves in to a back three in that phase of play.