If you're already doing shoot + kick + knife, you're ahead of the game!
But the case at 3-1 isn't the last one. I think you'll get the final upgrade at the end of 3-3. I usually skip the one at 3-1 because I know the last one isn't that far away. You had no idea though, and I still think it's a good purchase.
Also, have you beaten 3-1 yet? I ask because the last segment is kinda infamous.
I am stuck right now at this crank section. I am out of handgun ammo which isn't making it easy.