When you place the first unit, up on the the right it will say 1 of 1 click the arrow so it goes to 1 of 2 then to 1 of 3 etc. And then change the unit for each of the 3 to the ones you want.
I already have paid a lot. I own every module but 3. Why would I need to pay more?
I would gladly pay for a Dynamic Campaign, especially if it can be played in online multiplayer.