The gameplay in 2 is very different from 1. There is no inventory to manage and no equipment to customize. You collect ammo from enemies and from the level around you, instead of waiting for your gun to cool down.
You also get into the meat of the game much more quickly. The game doesn't bother with elevators or airlocks in between the places you want to get to. Action and roleplaying are better paced early on compared to the first game.
I only played 3 once, but I can tell you it's similar to 2, like most second sequels, it settles into a certain groove without many huge changes. But the weapon system in 3 is definitely an improvement. You'll be able to use any type of gun with any class, creating a tradeoff with how fast you can use your powers.