Here's a formation that should be considered illegal per your second rule (Some combination of at least 3 defenders (i.e. 3 players anywhere in the back row [DL, DC, DR] )) , but as you can see, it's not. I've considered recently trying a formation with 2 DCs and 3 DMs (DML, DMR, DMR) and I don't think it should be deemed illegal as well.

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