First of all, make sure you have read snYpir's Briefing.html tutorial and also his How to use Objectives and init.sqs.
Now, everytime you set an objective to "done", you also set a variable to true. You can give the variable any name you like - Zombie has used obj1 for the first Objective, obj2 for the second and so on.
So you will have triggers that look like this
Condition: whatever
On activation: 1 objStatus "done"; obj1=true
Create your ending trigger:-
Type: End#1
Condition: obj1 and obj2 and obj3
When the three objectives have been completed the three variables will be set to true and the end trigger will fire.