But that's only on stages that are small, with little room to move around, like Hades and Metropolis. I guarantee that levels like Sandover and Alden's Tower aren't going to be nearly as easy to land 3 kills on. Not to mention just plain bigger levels like the Dreamscape, or levels with water or constantly moving environs, like Franzea. I can't say much for levels that haven't been seen yet, like Paris, Columbia, Blackrock Stadium, or San Francisco, but Parappa's level 2 is definitely more situational than you think.
Depending on the situation (getting pounded and unable to land a kill), or the level, making a Level 3 more sensible, since chasing people down with no guarantee of the amount of kills you'll get and wasting 2 bars doesn't seem smart. It's all about the situation you find yourself in.
Another thing about screen-kill supers is that you have the ability to kill someone else's super. I don't know if it's the greatest idea to hold a 3 for very long, but if you hear a Radec popping a level 2 you can pop a level 3 and negate it.