The first two are probably in my top 5 games of all time. This one combines both into an experience closer to the first while adding new stuff.
Can't wait to boot it up later!
Edit to add some info:
The Risk of Rain series is now three games, all run-based roguelites. That means you start out with a set amount of characters.. Most of the cast still locked.
You start a run, collect items along the way, kill beasts, slay bosses and get bonkers strong.
Each stage ends with charging and defending a teleporter which triggers a bossfight and waves of mobs.
Depending on how well you do, what secrets (esp. in RoR2 proper hidden puzzles to figure out) you uncover and which challenges you beat, you unlock items for future runs, and characters.
The kicker of the formula to me is the difficulty mechanic: the more time passes, the harder it gets. Regardless of your progress im the run.
Depending on mode/modifiers you might choose to grind out the first stage, rush to a certain point for a basic set of items, or push towards the boss.
This much is true for all of them - though I haven't played Returns yet.
The first one is a pretty old-school 2D sidescroller/plattformer with a steep learning curve due to the limited mobility and static aim.
The second one is a 3D third person shooter/brawler depending on character. It features some more in depth decision making, secret stages, hidden unlocks, more challenge stages, character variations etc etc.
It's more modern style and fantastic movement make it a bit more approachable imo.
The third goes back to the roots. It's a 2d sidescrolling game but much more modern and polished compared to the first one. It features characters and items from both predecessors, as well as a host of new stuff to dig into!