You need to name all three objectives, here we call them 'target1', 'target2' and 'target3'.
In the trigger which checks if all 3 are dead you type in the conditions field:
!alive target1 AND !alive target2 AND !alive target3
In the onActivation line you type:
targetsdead=true; publicVariable "targetsdead"
If your mission is SP you can skip the publicVariable.
Now to the end mission trigger:
Place the trigger with a size big enough to hold your team - 100 meters should be enough. Group (F2) the trigger with your team leader and change the activation to 'whole group'. This activates the trigger only when the complete group is inside.
To check if both the group is present and all 3 targets are dead you write in the condition line of that trigger:
this AND targetsdead