I can only speak to my experience with MS products.
My first generation MS wireless elite combo wouldn't coexist in the same room as a wireless explorer 3.0. Both computers would experience strange mouse behavior. Random jumps of the cursor when the other mouse was being moved. The wireless elite was retired and replaced with a later version of a 3.0 explorer mouse and wired keyboard and all is fine.
The mouse from the combo set also wouldn't connect to the wireless base from one of the explorers.
In short, multiple Explorer 3.0's are working just fine, all within a 16x12 room.