Well, getting the list of discovering units is tough.
In your first trigger, in the activation field, put playerDetected=true.
In the deactivation field, put playerDetected=false (hope this one works, not sure).
U need to make a second trigger, trigerred by the detecting guys side, repeatedely, and in the condition field, put :
this && playerDetected.
Then I'm afraid u need to make a script to create a list of the detecting units :
create in your mission folder a listDetectors.sqs , edit it w/ notepad, put in it :
_nbUnits=count _this
_i=0
#Loop1
_u=_this select _i
?((_u knowsAbout player) >= 3):goto "onDetection"
#Back1
_i = _i + 1
?(_i < _nbUnits):goto "Loop1"
exit
#onDetection
;put here the actions taken when detected
_u globalChat "I know where you are!"
goto "Back1"
I can't think of easier solution
Whis'