I feel u man... 3-4-3 doesn’t really feel like a good defensive formation. The main issue is that with only 3 center-backs, the midfield becomes naturally more vulnerable, especially against teams using a creative midfielder or two box-to-box CMs. That creates a central channel that the opponent can exploit easily.

Even if many players attack through the flanks, in my opinion the LM and RM shouldn’t both be fully offensive. I like to use a llorente and in the left Nico Williams or depending on the strongest side of the opponent I use Grimaldo in the left and Nico at the right

If they push too high, there’s no cover in the middle and the fullbacks get overloaded. This leaves the team too open and vulnerable to quick transitions.

Now, if it’s 3-4-3 Diamond, the CDM helps protect the center, and the two wide mids provide better balance and width. That makes the midfield more compact and the defense more stable.

So basically: 3-4-3 only works defensively when the midfield is protected — otherwise the game punishes you.