Wow, you guys are neglecting the single most ingenious idea.
The third race is genetically augmented WW2 Wehrmacht soldiers, transported through time to combat the alien menace. As far as storyline is concerned, the Wehrmacht successfully eliminate all aliens, then proceed to put all ethnic and religious marines into concentration camps. Then Herbert, the one dretch who survived without the Wehrmacht noticing, sabotages the German execution machines to free all the marines. Then the commanding Marine, Sgt. Smith, lives a wonderful life having wacky adventures with Herbert. There's a television show spinoff, and millions of dollars worth of Herbert and Smith merchandise. They'd even have their own theme song.
"Ohhhhhhhh ittttttt's Herbert and Smith, Herbert and Smith, the wacky adventures of Herbert and Smith,
One eats with a mouth, one absorbs nutrients through a membrane,
And together they get revenge on those evil german Nazis!
One's got two fists, the other a poisonous tongue-like appendage..."
.....
Shut up, it's a good story.