Hi,actually 4-3-1-2 is quite similar to a 4-3-3,but best performant against dc-dc-dc opponent defenses.
Last season,I used this 4-3-1-2 when i had to recover a gap,i couldn't recover using my default formations (4-1-3-2,5-3-2...),and I succedeed.
This season i've your same problems with all these formations,since I noticed a general regression of my team in defensive attitude. So I chose to set a counterattack formation,with many attacking players placed on my opponent area:
4--MC-MC--AML-AMC-AMR--ST (4-2-3W-1 formation),caring about a good defense to stop opponent's counterattacks.
About your situation,you should:
1) Try to protect your defense with a 4-1-3-2 or 4-1-3-1-1
2) see the reason of your 3 mc bad performances (not in their preferred positions ,or demotivated) and if changing formation may be useful for them.
If their performances are constantly low, despite of all changing you make to your formations and their positions,maybe they're no more matched to the level of your competitions (technical or motivational reasons).
I had something as 5 mc, 3 of them often had 4 or 5 as performance evaluation (2 of them were playmakers).
So I sold one playmaker,and another Mc, and I bought 2 other MC.
From various friendly matches,against stronger teams,this new schema seems to have improved attack and Midfield performances,and this also maintains the clue of the game,far enough from my penalty area.
Of course i don't know the skill levels of your players,so i can't say if they could be suitable to this solution.
This is a task for manager,since there is no formation that lasts season after season.
Good luck!!