Maybe you should try to specify the value for each GPU

eg: 3x GPU used parameter -strap 1@2100,2@2000,3@1900

it means 1st GPU used strap #1 with a memory clock 2100 Mhz, 2nd GPU used strap #2 with MC 2000 Mhz and 3rd GPU used strap #3 with MC 1900 MHz. You can play with strap index and memory clock values for getting the best hashrate and maybe avoid crashes. The higher strap index (strap 1) supports higher MC than strap index 2 (You can learn it from readme.txt in your claymore folder)