you need 3 midfielders, it can be any of the yellow positions, also the defensive or offensive ones: DMR, DMC, DML or AMR, AMC, AML

another rule:
and it cant be more than 2 on each flank. I played once with

---------ST----------
---------------------
ML------MC--------MR
DML--------------DMR
DL----DC----DC----DR

and it was illegal (but i changed it after a few mins so no problem)