For the 4-3-3, you need a strong MC, MRL
But remember this ... In this game are the most important Team instructions.
If you play Zone marking, you need players who play well Zone marking (MRL,MC,DMC,DRL,DC).
If the opponent plays Zone marking, you need players who play well against zones marking (FC,AMRL,AMC,MRL,MC).
Similarly, if you play More Roaming, you need agile players who play well More Roaming. (FC,AMRL,AMC,MRL,MC).
If the opponent plays More Roaming, you need players who are well-defended against such modes of play.
....
So, this game is quite complex.
You have to make tactical and team instructions to his players.
But also, you have to predict how the opponent plays.