Not sure if the OP was more championing 3 subs or the 3D effect that placing a sub behind the listening position creates. My system has 2 subs but because I live in a small house I don’t have a lot of choice on placement. The recent addition of the 2nd sub behind the primary listening position was totally transformative to my system in all the ways the OP states. My soundstage became amazingly 3D at all frequencies - not just in the lower octaves. My mains have now completely disappeared and there is much more space between all instruments. One of the best additions I have made to my system as it is now much more involving.
Matt