You should be able to get two before they start running. Like Chris said start with the one that has the shortest escape route or go for the two that are closest together. When the third starts running aim for the legs. That will cause it to limp along. Lung shot on an elk is a pretty large target so it should be easy when limping.
Comments
Take a short second to plan the shots......
Start with the most difficult, and farthest....
Shoot slow and accurate, just a quick as you can
Hope that helps,
Chris