you can use a 3 speed rear hub from a lightweight bike on a schwinn middleweight frame,lace the hub to a s7 or heavyweight rim,if you use an s7 or other middle weight rims just use the caliper from a schwinn middleweight,if you want a front brake use a fork that has a brake hole init,if you use a ballooner wheel setup use a rear caliper from a krate or a 1020 dia-comp or a bmx of the right size,you can usually get light weight 3 speed bikes that havent been rode hard for a low cost,i just got one today with a sachs 3 speed with all the pieces still on the bike,freebe,if you have a frame that doesnt hav the bridge for a rear brake memory lane has the bridge to weld on a frame that they sel to convert a stingray to multi speeds,so youend up with a 3 speed bike for very little cost,TADA!!!!!!!!!!!