Or ... if you wish to play alone against comps, you "host" a pseudo-LAN game. I say "pseudo" because you can host the LAN game even if your computer is not connected to any other! Then, by default, you will be set on a team with 2 comps, against 3 comps. The game setup options have been disabled in this beta, but you can select one of the 3 demigod types for each of the 6 players (you click, or right-click, on each of the 6 heroes, on the battlefield image, and you get the option of taking their position on the field or of changing their demigod type).
P.S. > On another thread, a guy stated that he did a 2 v 2. I haven't seen if it was possible to reduce the number of comps : maybe it is.