Lets assume you mean you have 3 routers that are hooked together and run out to a single ISP connection....the other option 3 actual internet connections is just a more advanced form but it is more the question of how to use multiple ISP.
You want to run only 1 true router and that is the one that talks to the ISP. The other 2 devices you want to run as AP and connect back tot he main router. Only the main router will give out IP addresses and run NAT. You want to use the same SSID but different channels on the AP.
Now this only partially works. Windows is a very stubborn software once it connects to a wireless it will stay with it even if there are better options to choose from. Many times you have to manually disconnect it and then it will pick the strongest signal again. This is mainly used to prevent the PC from jumping back and forth between signal constantly. This parameter can be adjusted but it is a trade off, you either deal with staying connected to a poor signal for too long or you jump around which causes outages as the encryption is renegotiated.
One of these days I hope someone write firmware that will let you control groups of AP and hand off session like some of the commercial system do without loss of all the encryption keys.