Even as a rabid Undead supporter, I'm not so sure that 3 Undeads would be able to walk all over 3 Humans. All Humans bashing aside, and all "hostilities to your old race" aside (as if I had any in the first place...) Humans aren't something you put at the end of a wooden stick and start mopping your dirty floors with. They can be a formidable force, and have quite an impact at all stages of the game, though they seem to gain their greatest power in the early game and in the late game, mostly the late game. On the other hand, the Undead are a powerful late game race too. A 3v3 between UD and HU would come down to perhaps superior unit matchups/combos, stronger heroes, and a fatal error on the part of one side or another.