Honestly, I used one watch during my exam and it was fine. Just start using the watch early so you have a good handle of working it before going into the exam. Three watches might be a bit messy, in my opinion.
@AllisonM Holy cow...I didn't even think about a backup. I had one and it worked fine, but wow...thinking back, I guess I was lucky. I used that watch for 10 months before the test and never changed the battery.