It would be close to impossible to sex a Jag that is just 3", atleast going by the body markings
I have a 6" breeding pair (they have made 3 attemps at breeding this far - yet to succeed) that would appear females at times. But when they are guarding eggs, or fighting eachother their body patterns become very distinct and different from eachother
yea i'd say keed em seperated..... If you keep feeding them those large goldfish, like someone said before your chances of keeping them together would go down alot