I believe that (and in my experience this is true), that if your MC is helping out the defence, getting in tackles, making interceptions, then the 3-man defence will just sweep the loose ball. It makes it easier for the DCs to defend and not concede a whole lot.
But even if you find that the defence is doing too much blocking and throwing themselves in front of shots, then more often than not, your MCs will get a bad rating, because they are not providing sufficient shield for the defence. You can see this in the match rating when even tho you lose, your defence get 7-8 rating while your MCs get 5-6.
I played 3 man defences for two seasons now. When I lose, it's always because my MCs are not performing. My defence is rarely at fault.