I agree with Jan, you're doing everything right, I had to introduce 2 sister mice to another mouse I got rescued from my school(my teacher was going to feed him to a snake:O!!!)
I thought easiest to put the mice together on a large table. Luckily I had one of the sisters who was very explorative and accepting. Next step was to integrate them into one cage. I find easiest to clean a cage, put them all in(the loner first then the duo) and watch. becuz u see the two have each other to deend one another and the loner has no one. so he must go first to set a small area to himself. Slowly they'll get used to each other and even cuddle up! If not, you might just have to keep ur loner alone and ur 2 together. Making sure u take the single one out more than the 2.
