What is safe working load for a three sling pick

If you are using three slings you would still use the safe working load of a single sling as your WLL.
You are going to be in the range of 4200 to 5000 lbs for the systems working load limit. Now if you consistently use 3 slings they share the load. This puts less weight on each sling. So what you are really doing is extending the life of the slings because they don't experience the full WLL on each sling.