A 3 man defence should be fine. But i would not change my formation every match. For so important matches it is good if you play a formation your player fell familary. Sometimes a team needs a few matches to be good with one formation. So i would not experiement so much.
I think it is the best way to play a formation that you and your players play succsesfull.
But you can beat it with 3-5-2 with normal mentaly and counterattacks. If your opp plays defensiv you give him the ball and thats your chance to take away and score.

Good luck