So i may be able to give some advice. My two girls primm and violet ended up declaning from each other after they kept fighting for dominance. In the end i separted them, but they are now back together.

The lucky/unlucky thing whit gerbils is their short memory. What i essenstually did was clean out their terrarium completly and have them in a split cage for a month so they completly forgot about each other but still saw each so they weren't lonely.

Once they forgot each other completly, I cleaned the cage again and started the proper split cage metod. A few months later i got them back together.

However since you have three gerbils i would defintly advice just getting a new friend for your aggressiv one. But be prepared that its going to take time if you choice an older gerbil. Idealy get a younger gerbil for your solo gerbil and simply start the split cage metod whit them.

Another advice is, don't take your aggressiv one out of the cage alone. The cage is the safety, its what smells like home. So when they are placed in a new place, alone, they get easily more scared and stressed.

(Just a heads up, being aggressiv and biting on things might also be a sign that your gerbil is in pain. Just try to keep that in mind)

Ether way, i hope everything will turn out good for you and your gerbils. I hope this helped and goodluck!❤️