There is the potential for a lot of cancellation effect with even just 2 subs and your main speakers on the same wall... I would not add the third up there. even still, if you can individually adjust the two subwoofer outputs for level and distance, than you should be able to keep the front two on one output and use the second output to control the 3rd Sub.
Regardless, we all have to deal with the idiosyncrasies of our own rooms... be flexible and creative in placement!