I'll assume that most people don't really remember what the heck Rare's Killer Instinct even is, so here's how I remember it through my hate-tinted glasses.
KI was a pretty terrible fighting game with a ridiculous combo system that played itself somewhere around 25 percent of the time. The graphics were all pre-rendered, like Donkey Kong Country. It had finishing moves in it, which at the time seemed like a big ol' MK rip-off.
...
OK, fine, I'll leave my weird and probably irrational dislike of the KI series out of it. A lot of time has passed. The fighting genre isn't what it used to be. At this point, I'd be at least a little interested to see what's up with Jago, TJ Combo, and... uh... the robot dude? Or the wolf guy? Oh, hey, Orchid! That's the name of a character in that game, right?
Obviously, the existence of sheet music with "Killer Instinct 3" written at the top of it doesn't really prove anything. It could be entirely fake, it could be music from a game that's never coming out, or maybe they're actually going to follow through with a sequel. Which of these scenarios do you think is the most likely? We'll be watching our "time until someone who can read sheet music actually plays and records this" timer while you take all this in.
[UPDATE] In case you want to hear it, ChrisTilton has thrown the sheet music into a violin synth. Listen! Thanks, Chris!
[UPDATE 2} Continuing the epic tear that only a legitimate game soundtrack composer can touch, Chris Tilton spent some more time with the track and fleshed out what he thinks the full song might song like. Neat!