Open notepad, put in the code below & save as random.sqs (not random.sqs.txt)
After that put a unit (player) on the map, save ur mission & in the missionfolder put the random.sqs
Put a trigger Radio Alpha on the map, make it repeating and in the activation box put [] exec random.sqs
random.sqs
?(!local server):exit
_pos = [(getpos player select 0) +5, (getpos player select 1) +10, (getpos player select 2)]
_ran = random 3
? _ran <= 1 : goto "tank"
? _ran <= 2 : goto "chop"
? _ran <= 3 : goto "man"
#tank
"M1Abrams" createvehicle _pos
exit
#chop
"AH64" createvehicle _pos
exit
#man
"SoldierW" createvehicle _pos
exit
_pos can be anything you want though. For example a marker or a gamelogic...