I dunno. Based on those graphs Randy is clearly getting smoother FR from multiple subs. The SMS graphical display is limited to 1/3 octave spacing so its not really as smooth as it looks. The null at 45hz with the 600 up front would make the single 600 sound anemic for front row seats during action flicks.
I would suggest using the natural rolloff of the 350s to your advantage and corner load the pair up front to boost low bass output and flatten that 45hz null, then either leave the 600 where it is or move it around until you find the best placement. One advantage of leaving the dual 350s up front is less problem with getting them in phase with each other and the mains. Its far easier dialling in one 600 as you move it around. If you can try to keep the 350s at an equal distance from where you are measuring and the 600 at a distance of the 350s less 3ft to get the time delay aligned between the 3 subs. The bass will sound tighter.
