It's possible to have 3H/2J on one account (you would have to buy the 3rd Hopper), but they won't be in the same system (meaning they wouldn't be a whole-home setup). Currently, we only have a duo node which will control 2 Hoppers, and their corresponding Joeys. The last Hopper would be connected to its own node, and would not integrate with the other 2.
There is currently no word on whether or not a trio-type node is in the works. As you may know, Hoppers do not currently have the ability to communicate with each other at this time, but they will.