Let me rewrite it:
MacNeil quickly said, "But we don't know what's going on there. Anyhow, it's not our problem. Australia is. Assuming existing GDI forces haven't been able to drive Qutar back"--he grinned-- then the 22nd gets to do what it always does."
Captain Henry said, "Sleep in after morning bugle has sounded?"
Hastings laughed. "Guess again."
Captain Henry said, "Throw another shrimp on the barbie?"
Captain Hastings didn't even crack a smile. "Third guess lucky maybe?"
Captain Henry said, "Pull other people's arses outta the fire?"
Hastings almost smiled. "Took you three guesses, though, Captain."
But he didn't need three guesses in the original version - he got it in one!
It means you come up with an explanation or a guess at something, and get it right the first time.