The _easiest_ thing to do would be to grab a spare computer with four NICs, toss IPCop onto it, and then use it for the three different networks (one for each floor of the house). So, on two of the NICs, you'd have cables leading to the wireless access points (use channels 1 and 11) for each floor - give them separate WPA2 passwords and SSIDs to avoid confusion. On the third NIC, you'd have a switch with all of your networking gear on it (use channel 6 for your own wireless gear - this will minimize interference). IPCop has support for QoS and packet shaping, and, trust me, you'll want to use them.
However... I would discourage you from doing this. For one thing, it forces you into the role of sysadmin. Wireless goes down? You're responsible to fix it, ASAP (and if you're not, well, how much are they really going to pay for it?). Another problem is the legal liability - your tenant downloading child porn is going to have an IP with _YOUR_ name on it. Finally, unless you've got access to a seriously fat pipe, your tenants are going to be eating a lot of your own bandwidth.
I appreciate the sentiment, but this is not really all that great a plan.
-Erwos
