Referring to another answer of mine How to escape Fort Zancudo without getting wasted you're going to have a very difficult time raiding the base. The core issue is that while you can enter the base via vehicle you cannot exit by car (or tank). The gates do not open from the inside.
You'll need to exit the fort via aircraft and there are only three that spawn with enough seats for the Michael, Franklin and Trevor: The Buzzard, the Annihilator and the Cargobob. This is a dangerous task because these aircraft don't have a guarantee of spawning in the first place unlike the P-996 Lazor jet plane. Likewise, these vehicles are exposed on the airfield and take time to spool up before taking flight.
Of the three the buzzard attack helicopter is your safest bet to escape: it has the shortest idling period and strongest armaments. After entering the aircraft you must wait around five seconds for the rotors to spin enough to generate lift. During this time you will be susceptible to incoming small arms fire, tank shells and homing rockets.
The last obstacle in the plan is the fact that you will be unable to switch characters during this raid. Despite all three characters being together you still cannot switch characters while you are wanted (story missions are exceptions). That means that if you die while playing as Trevor you will be unable to switch to Franklin.
In conclusion, this is a very risky operation and there is not much in terms of rewards. The minigun in the air traffic control tower is better than nothing, but even acquiring that is exceedingly difficult.