The biggest change is in the graphics. Not only does AH3 look better than AH2, it also moved the load from the CPU to the GPU. That has been somewhat of a shock to those who could run AH2 on a home office laptop. The current version needs a gaming level computer - just as almost any other game in the market! However, you don't need a top notch gaming monster, AH3 still is reasonably moderate in its requirements.
What else? Whiners still whine, Main Arena (now called Melee) is still the most popular one despite the totally free ones, scenarios still get their rosters filled... Some minor changes here and there but if you're mostly into aerial fighting there's not much that has changed except a new default FOV which makes the planes bigger in your crosshair.
Try the free arenas to see if your system can handle the game and if it does, reactivate your account.