If you watched Sonic 1 & Sonic 2, it's a no brainer to watch Sonic 3. I think Jim Carrey is the best part of all three films.
Additionally, the story loosely follows the 3D Sonic games storylines, so if you grew up with those games it's fun to see them come to live action.
If you have no attachment to Sonic or his games, you'll still enjoy some parts, like Jim Carrey, but otherwise you've just got to accept the premise and be fine with whatever is happening.
If your main draw is Keanu, I mean he's in the film, but he's a talking hedgehog so... You know... Set expectations.