If they are still fighting after you have done this about three times then it probably won't work. Some gerbils, especially some females, will never accept another gerbil once they have been living alone. Males are very protective towards smaller gerbils so introducing a lone male to one or more 5 week old pups is usually very easy. If you are doing this you may not need to wait three days. One day of the split cage method may be enough.
Females can be much more aggressive than males.
Give them longer to get used to one another. Never introduce a gerbil to a group of adult gerbils or try to introduce two groups of adults. Fighting is almost inevitable. Because females can be so aggressive it is usually best to keep females in pairs whilst males can be kept in larger groups without much trouble. Hopefully, by following this advice you will be able to pair up lone gerbils safely and give them the companionship that gerbils need as social animals.