If you added the three new mice to the existing two that were already in the cage, my first thought would be that the already established group killed the invaders. I've found that mice can be very territorial and can kill with no apparent reason. When I set up breeding groups I always use a freshly cleaned tub and add all of the mice that I want in the breeding group at the same time so they all find themself in a new environment together. Then I never add new mice to the group and retire the entire group together after about three months. A lot of the times when I've added new individuals to an already established group I've had problems.