Playing with 3 defenders(suggestions appriciated)+ thoughts on unrealistic formation

Hello everybody! Another long post by me! :D
I decided to start playing with 3 DC since I always had a hard time signing flank or wing players both in DF and MF so I decided to worry only on the midfielders and play with 3 DC. Even thou this type of formation is very popular in real football (ex. Chelsea, Juve etc..) in this game its not the best and I payed the price in the last seasons. LB and RB are awesome they score goals and give assist while with DC you are lucky if they get one assist or goal the entire season! The team seems to be vulnerable especially down both flanks! Any tips what I can do to improve this by still holding the same formation with 3DC.
Another strange thing that I find unrealistic in this game are players playing with three defenders LB DC RB. The purpose of the fullback most of the time is to give a push in attack. You are basically playing with one 'real defender' the DC. Yet I see teams in my level, who are weaker than mine use this kind of formation and have a lot of success. In real football when a team plays with three defender they all play in the center back position.

Sorry for the long post, and a big thanks in advance for reading!!!