Ok, uhhhh...Orks are aggressive and inelegant swarmers that want to close in and overwhelm you. Eldar are fragile speedsters and hard counters that demand they go in their right place at the right time. Space Marines are tough elites with a jack-of-all-trades-master-of-none concept aside from their small numbers.
Like I said, I haven't actually played DoW3 though, so that's about all I can give ya. Stuff like their actual power curves are something I can't assume.
Gotcha