I'll second vircom. Had my (working) Superhub 2 swapped out by a field technician who wouldn't let me keep it, and the Hub 3 is utter, utter junk. The only circumstances where you should swap a SH2 for a Hub 3 are where the SH2 is broken or failing to work.
The only technical advantage is that the Hub 3 can support 300 Mbps (think the 2 tops out at 200), but in return you get all the faults related to the Hub 3's Puma 6 chipset.
I was having on and off problems with the Super Hub 2ac which is why I switched to the Superhub 3.0. I'm really happy with it but I'm well aware alot of people are not. If you're SuperHub 2ac works ok then I recommend you stick with it until the 4th Generation Superhub gets released.