This was taken from
www.theknot.com:
"Q. One of my bridesmaids has dropped out of our wedding party. Is it okay to have two groomsmen walk with one maid?
A. Having two groomsmen escort a bridesmaid, one on each arm, is completely acceptable."
I couldn't find anything about having an uneven number standing up there with you though. I'm sure whatever decision you make will be just fine
