I've finally built all 3 star fortresses and messed with them in combat (generally by waiting for an invasion and seeing how they do without support). My brief notes:
Generic- they all get drones (little orbiters) at level 3 and a minefield. The mines seem to be functionally identical but visually different. The drones will often smash into enemy and allied ships.
Hightech- legit. Not much to say other than hightech stuff is good especially on a station that cares a lot less about flux.
Midline- Surprisingly good. I thought it was going to suck due to its shape (maybe the lower stages do) but the level 3 is a monster. 4 mirv missiles on the sides (2 on each) with the "doom side" sporting gravaton beams, auto cannons, Hephaestus, and Mjolnir will rip through anything and everything. The back side only being the hanger and some heavy mortars isn't bad at all at cleaning up stuff. The higher spin rate isn't bad either since if you can wall off the sides it lets it vent flux each go round.
Lowtech- feels off. First of all, it's got flack for DAYS. Flack cannons + vulcans + devastators. The issue i feel with the low tech though is that it has 2 hellbores, 2 reapers and I think 2 heavy motars on one of the arms...and thats IT for non flack/shield damage. The whole thing basically doesn't deal HE damage 90% of the time, and instead grinds through everything with an absurd amount of vulcans. I really feel the fort could use some light motars/assault guns because it's silly to watch a kite throw up its shield point blank and just tank the entire thing.
I'm also a little surprised that the lowtech doesn't get teh hephaestus cannons and is stuck with just mark IX's/devastators. Having some of the worst options for bombers as well (yay..piranha) leaves the station in a really weird spot.