well, you could just leave it as is and only use one of the chainrings, but that's ugly and heavy and lame. if you feel like one of your existing chainrings is the right size, then take your cranks apart, remove the other two, and replace the long chainring bolts with single stack chainring bolts. otherwise, buy a new chainring of the appropriate size, and make sure it will fit your existing cranks. the cheapest ones are from rocket for $18.

fitting brakes on the smaller wheels will be a problem. there's not much (that i know of) that you can do, aside from keep hunting for a brake with long arms. ask your LBS, they might have some old used brakes that may work.