Sure, different triggers should be quite good enough (alternatively a simple script, or a procession of gamelogics with waypoints). Also consider changing the flavour text in your briefing.html and making each ending a different END# so that players can see the effects in the debriefing as well.
I suggest you make the "leaving the forest" trigger a general one that just turns a global variable like "EndChat = true" true, and then add the extra triggers:
For 1:
Condition: EndChat && {alive _x} count units group player >= 8
On Activation: <good work chat>
For 2:
Condition: EndChat && {alive _x} count units group player <= 7
On Activation: <good work but your team is dead chat>
For 3:
Condition: EndChat && {alive _x} count units group player == 1
On Activation: <you screwed up!>
That should do it. As for the different endings, I'm not sure what you mean...it's the end of the mission: shouldn't they all be evacuated from the same spot, no matter if there's one or 10 people surviving? What I suggest you do, though, is make the final trigger a "Switch" trigger, and synch it to the regular end waypoint trigger that leads to the 1 & 2 ending spot. Then add another WP after the regular last waypoint, which would normally not be seen, but thanks to the synched switch wp, it'll switch to it instead of the regular one. If you catch my drift? Alternatively, just add a new task with a new task WP, since that's the way to do it in Arma 2
Hope you caught my meaning here either way. Good luck!
Wolfrug out.