Many of you stick too much in NUMBERS and trivia, while the question was a different one. If you want the same question with 3 units, have 3 units 60-30-30 or whatever else. You can also make it "4 axemen vs two archers, 60-60-30-30 odds, or 6 grenadier vs 3 riflemen, 60-60-60-40-40-40 odds". My point is to see how much is it reasonable to risk.
And no, you can't wait for catapults - they are about 2000 years away for this scenario. Also don't stick in specific units: suppose it's Modern Armor age but the situation is same: imagine you have two Modern Armors vs one, with odds 60-30, and you want (for whatever the reason you think critical - perhaps because you want to save that town) to attack at THAT turn and you don't have any more Stealth Bombers around. Which way do you go?
As correctly stated by some, AI is not afraid to start with the low odds units - but I wonder if this is a correct policy (that's why I posted the question).