This works for SP, don't know about MP though...
You should set variables for every objective. For example when you finish first objective variable
Objective1 equals true (Objective1=true). Repeat this to second and third objective. Then you need fourth trigger which has condition:
Objective1 && Objective2 && Objective3
The trigger activates when all three objectives are set to be true, in other words, completed.
Should these variables be broadcasted with publicvariable