Bonded ADSL lines are usually provided by your ISP, but networking company Managed Communications can provide this without the need for switching provider.

Up to 16 SDSL, ADSL and Max lines can be bonded, and rather than using the traditional method of Multi-Link PPP (MLPPP) they use a virtual tunnelling protocol creating MPVs (multi path virtual private network links) which link back to their central datacentre infrastructure.
All that is required at the customers end is a speedtouch ADSL router and a XRIO bonding device, with an enterprise carrier grade bonding device at the datacentre.

As this is obviously targeted for business use so there are no prices available, but one thing which comes to mind is that if you bond three ADSL Max connections then maybe two of them will be connected at any one time without dropping!!!