In the trigger "type" pull down menu, you can select win1, win2, .... win5. I have not experimented much with the different win options, so I am not sure how you would use them. But if you are not doing anything advanced, just put something like this in the On activation field of triggers A, B, C:
Trigger A:
Condition : east_dead && west_dead
On Activation : hint "East and West are all dead. The civilians win!!!"
Trigger B:
Condition : west_dead && _civ_dead
On Activation : hint "West and Civilians are all dead. The East wins!!!"
Trigger C:
Condition : east_dead && _civ_dead
On Activation : hint "East and Civilians are all dead. The West wins!!!"
Of course you can change the messages to whatever you want. Or if you want to do some more advanced stuff, like run a different cutscene you could call a script that does the appropriate cutscene for which ever side wins. Doing cut scenes involves advanced scripting, but there are nice tutorials in the editor depot on scripting and cut scene scripting. For now it is probably best to keep the simple ending, but once you varify that the overall mission works and the ending triggers work ok, then latter on you could add more advanced stuff like ending cutscenes (they are called "outros"). To call a script from the On activation feild you put something like:
[param1, param2....paramN] exec "myscript.sqs"
param1, param2 ....paramN are the parameters that you "send" to the script. The script called "myscript" in this example is a script that you put in the mission folder. This is all explained in the scripting tutorials in the editor depot.