In reply to liquid:
> A chain stretches overtime, which causes wear on the cassette - having three chains on the go seems like ball ache rather than changing a chain as is needed but essentially you are just reducing the amount of wear on anyone chain so it would extend the life of the cassette ultimately.
I'm a complete numpty when it comes to bike maintainence, but swapping chains is so easy that even I can do it. I do mine every 200 miles, and simply have 3 sandwich bags marked 1, 2 and 3.
Clean the chain - and you probably need to do that more often than 200 miles - then find the power link and unclip. You can get a tool for this - odd looking pliers - and it's worth it, and was pretty cheap anyway (possibly from planetX). Off comes the chain, into the bag, next one goes on.