In a blind test, people fail to hear a difference between an Axe FX 2 and a real amp. Also, they fail to hear a difference between an Axe FX 3 and a real amp. It logically follows, that it's hard to hear a difference between Axe FX 2 and Axe FX 3.
There is a difference, sure. Axe FX 3 has more oversampling (I think?), and there were a few improvements since the firmware freeze for Axe 2, but to call the difference between the two anything more than "Slight" would be nothing more than an exaggeration.