Yes it is better if you have 2 midfielders they better be MCs... But even in 4-3N-3 and 4-3W-3 there are 3 Mcs so there is no danger in illegal formation unless you mean 4-2-1-2-1
If in 4-3W-3 your MC gets a red you're left with 2 midfielders and they are not MCs. That's bad.