what's a legal module?

The 3 defenders are quiet better than the ST's of the opponent.
What is the way of thinking to determine if a team-formation is good or not.

My idea (i'm a noob): 3D's + 1 DMC can cover the 2ST
On the midfield, I have 1 player extra, so a lot of ball possession.
And 2 ST in attack. (I think AMC + ST would be better, then his LB en RB are useless)

Where is my thinking wrong?