Just updating this thought experiment since it is long since we have better stats on weapons.
--Lets presume the same 3 Vanguard attacking--2 Hoplites and 1 Sentinel.
--Lets presume the same StarG has upgraded shield and the attacker still needs to drop 6,000 points shielding for their spike missiles to get through.
--Lets presume the Vanguard all load the M6A laser on the swivel for taking down shields. They can now do this at 5 km range, safely outside almost all weapons ranges, save missiles, and StarGs carry no missiles. Note the mistake in the official tables says the StarG does carry S3s (fixed instead of pylon), and we don't know for sure there will be none. Table update should be soon. We do know you can't fire missiles without a computer.
--Lets presume all the missiles on the Vanguard are spike. Together they can fire 4 salvos of 4 S2 Arrestor IIs, and 2 S3 Arrestor IIIs. Also note the cycle and lock times on the Vanguard are all top self. Vanguard are mighty as missile platforms. It is very likely that even with counter-measures, every missile in the first salvo will hit and disable the StarG.
--There is no need to stipulate Vanguard weapons further except to note they may be able to load Pyros in their turrets and shoot down incoming missiles.
The StarGs oversized shields will come down in 4 seconds. Their computer, propulsion, and unmanned turrets will drop a second or two later. About 5 seconds and the ship is defenseless except for the 3 big manned turrets, and any escort. Any escort with a chance of saving the day will need to be able to fly into a storm of laser fire, that will only get much worse once they close distance so their weapons will work.
IMHO, no StarG can survive this sort of conflict unless it loads long range weapons in its manned turrets and on any fighter escort. I don't see any other way to keep ships safe, save keeping them away from dangerous space in the first place. I think players discount the issue of long range combat to their peril, especially when flying large, slow targe. . .I mean ships.