Short answer- I would place the 3rd sub center of the back wall as a default.
I knew you had an AT screen, so hiding as much stuff behind there as possible was my intention.
The studies at Harman have confirmed 2 opposing subs give the smoothest and most tactile LFE response for a group of listeners. What I found opposing subs lack in my own tests is a natural bass presentation. It always feels like the bass is coming from under you like an earthquake. I like the bass to couple to the images in the soundstage. A stand up double bass should not feel like split between the main loudspeakers and a nebulous place from below. I stole the word nebulous from Grimani. Like it.
My opinion anyway.
