Nothing wrong with him playing only a back three, though, if that's all that's needed, less skilled defenders stop my attackers all the time. He has a 4* MC in his reserves, could easily play 3-1-4-2.
You said something good but IMO he don't have such a strong team, it would be better if he would quit that MR and add the MC 4-1-3-2
Midfielders to be like ML - MC - MC, it won't be any problem for him as long he have an DR