Fish Herding Behavior

Ok...I've read the posts here regarding pregnant fish and corresponding behavior but I still can't quite figure out what's up with my fish. I have 3 small Comets (cheap goldfish) in a 5 gal tank . For a month they've been fine but now one is herding the other two...not biting or chasing, just herding. I figured this might be mating behavior since the two getting herded were also looking plump. None of them have white specks on thier gills or gill covers. I removed the troublemaker, put it in the original fishbowl and set the bowl next to the tank. There is peace now but several times a day the fish will stick near eachother even in separate tanks. Is this mating behavior and if so, how long do I wait before I can put them all in the same tank again with no babies?