This is definately possible and is often used in enterprise as "Bandwidth on Demand" where the main router has T1/Frame Relay as primary and ISDN/DSL/Cable/Dial as backup.
In cases where the T1 reaches a certain utilization (generally about 75-85% in Frame) lasting over a certain period of time (meaning high usage not burst), the ISDN is invoked and bonded to the T1 as a manner of load balancing/sharing.
The tough part is figuring out how to establish the criteria for turning it on and more importantly, off as most business ISDN is billed per minute.
If you are thinking of having 3 sources simultaneously and bonding them for aggregate bandwidth, I don't think it will work without serious load balancing equipment on your end like a F5 BigIP as you lose sequence, timing and path from source to destination when traffic to one site is passed over multiple networks.
If you want to make a fast network for a whole bunch of users, aggregating multiple low bandwidth providers, the way I would do it would be to have each source feed a dedicated proxy server and then use either another server or dedicated device to get the servers to work together as a cluster, sharing a single IP Address across all the LAN side NICs of the proxies.
Remember in any case that even if you have a T3 at home your browsing speed is limited by the weakest link (bandwidth, latency, server load, hops etc.) between you and your destination. Like driving a car, you don't get to your destination any faster on a 10-lane road than a 1-lane road if you are the only car on it.
I hope I have come close to describing what you are envisioning...