Sounds to me like you should leave well enough alone if it's working ok at the moment.
The reason for having longer cages on rear derailleurs is to take up more chain slack as you shift between gears and have differing numbers of teeth in use on the chain rings and cogs. RDRs with longer cages have a greater "capacity" which is defined as the [largest chain ring teeth - smallest chain ring teeth] + [largest cog teeth - smallest cog teeth]. Your mountain bike with 3 chain rings will need a RDR with a larger capacity than the folder which only had a single ring. It's not just a question of shortening the chain since if you shorten it enough to work in the (small chain ring and small cog) combination then it'll be too short to handle the (large chain ring and large cog) combination.