Mice/Mice are terrified, and mice fight.
I have another question and since my other one was so long, I thought it would be easier to send them separately.
I bought 3 mice 5 months ago and I can't seem to tame them. They don't like to be picked up at all. Sometimes they will run when I come near; sometimes they sniff my hand and run away. They will only crawl in my hand when they are in some situation they don't want to be in (like a temporary box when I'm cleaning their cage). Sometimes I get frustrated with this and just want to hold them and pet them because they're so soft but they get frantic in my hand and don't seem to calm down and so I put them back. I think I may have ruined them by picking them up when they didn't want to be picked up and now they just run from me when they can. I see videos on YouTube of people with mice who just sit in their hand and eat or like being petted. I thought holding them even when they don't want it would allow them to get used to me. Is it too late to socialize them?
I will answer both questions here, and refuse the other.
You must be patient with the mice. They are each being mice the very best they can. They are not to blame for their behavior. They each have both a personality and a past. You are buying mice from a pet store which does not socialize them. I would not buy from this pet store again. They should at least be somewhat easy to handle once picked up, before they sell them. So the mice are dealing with their past and present as best as their instincts tell them to. And they will feel it if you are angry. Forgive them. They are not to blame. Even the aggressive ones are not to blame for their aggressive behavior.
You do need two cages. You must never keep mice together if there is blood. You have done everything possible to try, and it is not going to work. The third mouse will be absolutely miserable her whole life, if she lives with the others. And... You should buy her a cage mate too. This situation happens, and you have to go with the flow of the mice's needs. It is truly unfortunate that this mouse situation is extremely inconvenient for you. Her new friend should be a little baby. It is always best to introduce a baby to an already established group or even a single mouse. The cages should be very close before you get the new cage mate, so they can communicate, which they do in high frequencies which we can't hear. Otherwise the third mouse will be lonely and sad, and susceptible to depression, illness, and mites, as all single mice are.
As for taming a terrified mouse, I detailed very careful steps at the following link. It is a long answer, but please read it all very carefully. When you are following the steps, you must repeat a step over and over until she reaches the goal of the step. This is a long job, but should work if you have the patience to do it. If you don't have that patience, you will just have a terrified mouse. There is no faster route. But she may be a fast learner. You just have to go at her pace.
I wish you the best of luck and as fast success as possible. You can do it if you are patient and forgive every time a mouse does something you don't like.