The answer to this question depends mostly on this... (also, I'm assuming that, similar to the Axe-Fx II, the Amp blocks in the Axe-Fx III have their own devoted processor, and everything else runs on processor #2)
If the Amp block, for the entire lifespan of the III, can operate at full resolution + anti-aliasing without using more than 33% of its processor, then yes, a third Amp block would be much appreciated.
If Amps blocks will operate at reduced resolution / reduced anti-aliasing if a 3rd is enabled, but will still operate at 100% resolution / anti-aliasing when 2 or fewer Amp blocks are enabled, then a third Amp block would still be much appreciated.
If adding a third Amp block will reduce or impede the resolution/anti-aliasing/future improvement or development of the Amp block overall, then no thanks.