I recently got 2 mice to be companions for a mouse that had a companion up until she recently passed away. One of them gets along fine with my original mouse, the other has been attacking and chasing around the other two (Almost non-stop if they are not sleeping) they are in a large aquarium and have plenty of hiding places, food and water... The mean one has red eyes the other two have black eyes (does that make a difference) I don't want any of them to get hurt but cannot get a second cage, is there something I can do or should I try to return the bully?
There are unfortunately no easy answers and no sure solutions. So, before I start a suggestion, the question is how long the pet store will give you to figure things out. Will they give you a month? And also, the other two will be going through some stress in the following solution. You have to decide if it is worth it.
Actually, before we even start, if she really isn't a very special mouse, even an unfriendly one... You might as well trade her in, right?
So let's assume the pet store is willing to wait, and she is a mouse you want to keep. We'll examine the best possible solution. I am hoping that when you say you can't get a second cage, you could still get one of those small plastic tanks that pet shops usually carry in a number of sizes. If I recall correctly there is a miniature turtle size, which is of course way too small; and then there is one about a foot long and one about two inches longer than that. Even the foot long one would be fine.
This is a little counter-intuitive but here goes. Put the two submissive mice in the small cage for a week! Do you have one of those tiny mesh wheels that they can use? That would be best. And a couple of toilet paper rolls. After a week, the tank should smell wonderfully (to them) mousie. Now take out the wheel and everything else except the water bottle. Not even a food dish; you will put the food on the bedding. There should be one obvious place to nest. Now put the aggressive girl in.
If she is still merciless, I don't think there is any way to solve the problem. But if they get along after two days, with no blood and no depressed mice, all sleeping together, leave them in there for a week. Oh what a boring life! They have nothing to do but cuddle. When that goes well, put the wheel in. If they tolerate that, it is time for the big tank. Since the aggressive girl was just in there for a week, you want to make sure it does not smell like her. Wash with something with a strong smell such as Dr Bronner's peppermint soap. If you want to use a light bleach solution, rinse it so carefully. That could irritate their sensitive noses. All the toys, etc, must especially not smell like her.
Now put them all in the big tank, again with no toys (no wheel) and nothing but a water bottle and one nest and a few toilet paper rolls. If this doesn't work, again, I see no hope. If it does work- no blood, no depressed mice, no one kept out of nest- you are doing great! Some chasing and squeaking is always fine; just not nonstop. Then you have to add other things in slowly, always waiting to see how things go. Start with TWO wheels (I assume you had one before you got the mesh one). Then a few toilet paper rolls, the more the better (3-4). Then the other items one. One by one.
This is the best chance your girls have of getting along. If they don't - oh dear, now you have grown fond of her! She could actually live in the small cage as long as she has the small wheel. It is not ideal but probably better than the pet store, unless she has unusual markings so someone will definitely choose her as a pet. I fear that the pet store might feed a returned mouse to the snakes. You would have to trust them.
This is all the help I can give. I know I didn't make it easy. Best of luck.