Because the electrical wiring on each home is different, it's difficult to answer the question. I would go with one at first, connect it to the router and since the 722 receivers have homeplug built-in you may not need another two. If your receivers do not get the IP address, then you'll need to buy one for each receiver.
Plug it directly to the wall and test. Power surge protectors and UPS devices can block homeplug to the network. Another thing I found out is that you also want the receivers plugged in directly to the wall for this to work.