I had 3 Diono Radian RXTs across in my 5th gen but all forward facing. Never had them rear facing. Went from 2 rear facing Cybex aton2 infant carriers in the outboard seats and a forward facing RXT in the center because of space needed for the front seats. After they outgrew the cybexs, moved to the 3 forward facing RXTs and had plenty of room.
Like you describe would work, but depending on the front seat positions. Also accessing the center seat would require the child crawling under the rear facing dionos like a tunnel, which my oldest loved doing at the time!
Go find a baby store like BuyBuyBaby (if in your area), they let you test fit car seats and seat combinations. I was able to try different combinations of seats/carriers in both of our cars to make sure they fit before blowing $1200 on 6 seats! Good luck!