I need help with the following training

[SlyAnimal]

If you play with 3x DCs, then you need to either:

Change to AMR/AML (and get rid of your AMC)
or
Change to ML/MC/MR

As you need to have 1x player on each wing, otherwise you would have an illegal formation.

So yeah, actually switching down to 3x DCs probably isn't a good idea (As you'd basically end up just directly marking up against their defenders, so you'd gain very little benefit, better to just keep 4x defenders).

Although swapping to DL---DC---DR and then having a 3rd ST or MC might work ok. But I'd personally be a bit hesitant in doing that too, as I like to always have a full cover, rather than a half cover, on opposition strikers.