Quarantine the new female if you haven't already done so.
Remove the male and female from their cage.
Clean the cage well, wash the "furniture".
Rearrange the stuff in the cage so the configuration is different and so there is more than one choice of hide for the total number of geckos
Put all 3 geckos into the newly re-done cage at once
Watch them carefully. The male will try to mate with the new female. If this is something you don't want (or if she's not big enough), don't put them together. If there is immediate and dangerous attacking, beyond the usual mating stuff, separate them. If, during the course of the next few days or weeks (or ever, for that matter), one gecko is losing weight, getting bitten or scratched, or if another gecko is constantly sitting on one of the geckos, separate that gecko. It won't necessarily be the new female either.
If your enclosure is 20 gallon long or smaller, don't consider it.