Unity prefab into 3dsmax or another 3d software

Hello everyone :)

I'm trying to make endless runner game for android and I'm kinda stuck because my prefabs have too much meshes so it's running slowly.

Do anybody here know maybe how to export prefab so that I could open it in 3dsmax and attach all meshes to a single one or maybe some tool for unity that can do it in unity?

Thanks for all help :)