I have put acoustic panels on side walls, but these are not direct by the side of the speakers, as they were put for first reflection points via the mirror trick.
The room is a good size, but L shape. On that portion of the L where I have my system, the distance between walls is 11’, which means that I don’t have too much wiggle room from the side walls, otherwise the distance between speakers would be too small (I.e. only about 4’, if I followed the 3’ from side wall rule)
I wish I could post a sketch here, but don’t see how.