Trigger:
Condition: as you like
On Activation: loon1 assignAsGunner jeep1; loon2 assignAsDriver jeep1; loon3 assignAsCargo jeep1; [loon1, loon2, loon3] orderGetIn true
Syntax not guaranteed. This also assumes that it is impossible for any of these guys to have died.
Alternatively, attach a getin waypoint to the jeep. You won't have complete control of who takes what position though.
OFP is in general "blind" as to its surroundings: it doesn't know whether it is in a trigger, script, waypoint or init field.