Summary:

Assault: These ships will take out your opponents’ damage dealers first. They go after Capital ships and their Escorts, then other Assaults and Interceptors before moving on to Support and their Guardians.

Interceptor: These ships will race to the back lines to take out Support and their Guardians First, then move on to taking out the largest threats.

Capital and Escort: These two ship types will act together, with the Escorts eliminating threats to the capital while the capital concentrates on other Capitals and Supports before helping its Escorts finish off any remaining Assaults and Interceptors.

Support and Guardian: Guardians will concentrate on eliminating threats to the supports who will sit at the back of the battle and provide bonuses to the fleet.

Starbase: Sit at the back of the battle and prioritize aggressive targets.

Upgrading: Presumably will not participate in battle and will be destroyed once other threats are eliminated

My understanding is that ship roles will be assigned automatically by the program, so that one cannot assign an inappropriate role to a ship in hopes of some tactical advantage.  (Based on a recent dev stream.)   The principal problem for the player will be deciding how many of each type is a good mix.

There is historical precedent for the current rule.  When fighters intercept escorted bombers in daylight (or the ground/naval analogues), two doctrines are possible, and both have been tried:

• Ignore the escorts, press your attacks directly on the targets.  Lesson: This was a loser (even discounting the broader fact that the intercepting side is generally already losing the war).  The most effective attack vs. bombers was the head-on pass, after which you must laboriously overtake them again.  With escorts on your tail, you simply die before or after 1 pass.  Against aircraft carriers, you drop your entire bomb/torpedo load in one pass anyways.  If you don't dogfight, then the escorts get onto your tail for free, and you're the turkeys at a Turkey Shoot.
• Dogfight the escorts first.  I think all combatants (in Earth's historical wars, admittedly with low tech) converged to this idea, probably because the alternative led to unsustainable losses.  At the very least, your fighter escort must melee with their fighter umbrella, and whomever has numerical superiority can hope to sneak some naked bootlegs through.  Then beyond the tactics, it becomes a logistics, organizational, and production/training vs. attrition duel.  This applies to ground wars, too, c.f. Sun Tzu, the Romans, and all ground campaigns since then: you always try to hit them where they're not, but ultimately you win a war by fighting a showdown battle on a field you chose, with as many advantages as you crafted, and you break their army and then rout them from behind.

Of course, in the decades of relative peace since the last major war, smart techies have applied their efforts to explore/undermine/reverse these doctrines, with mixed results.  First, modern war is so darn expensive that the escorted swarm has basically vanished: there has not been a war bad enough to justify the cost.  Hence we expend vast resources to neutralize or avoid those dogfights in the first place:

• Precision weapons means 1 plane with 2 guided bombs can be better than a 600-heavy bomber raid.  (Not kidding.)  GC3: All missiles are implicitly very precise.
• Stealth lets you elide the escorts.  They can't even find you to start the dogfight.  GC3: Omits stealth as an abstraction.  (Hmm ...)
• Standoff weapons means you don't have to risk your planes in the dogfight/ships in the gun duel/infantry in rifle charges.  Venerable B-52s don't have to penetrate because they can launch cruise missiles from safe zones.
• Drones are ... an inscrutable game-changer.

Of these, we can already identify that GC3's abstractions have no equivalent of the submarine (stealthy ambush weapons platform), the stealth bomber, the ICBM (on interplanetary scales), the kamikaze (ugh), or new technologies that we can't even assess, like self-flying drones.  That's fine; we can adapt to anything.  Jutland-like naval encounters between big dumb gunnery platforms is already complex enough to be fun for years.  (Also, be grateful that there are no submarines -- the defense protocol vs. stealth is the most tedious, mind-numbing, repetitive exercise in caution ever, which would be micro hell at this scale.)

I have some peeves (but I defer them until Beta 5):

• No multi-target fire control.  Even a GC3 Titan must fire all of its massive firepower on 1 target at a time.  That's ... playable, maybe easier to balance, but kind of nerfed.  Modern Earth ships can split their fire many ways.  They're already tracking thousands of objects to build an entire picture of the environment within several km.
• SE3: A ship defaults to targeting only 1 enemy ship per round.  (Exception: all point-defense weapons can freely target any inbound seeker.)  You may research and add Fire Control components that increase your # of targetable ships, up to 1+5 = 6 targets per round with a max level 5 component.  (Then you'd also need 6+ separate gun components!)  I actually liked this rule; it created interesting ship-design and fleet-composition decisions.
• SE5: All weapons target independently.  Same as having Fire Control Infinite for free.  I thought this was meh.  In practice, tho, it made zero difference because I think I never, not one single time, actually paused a tactical ship combat and told one of my ships to fire one gun each on 2 separate targets.

Looking forward to Beta 4!  We'll crowd-tabulate all permutations of starbase-vs-ship and hunt the breakpoints

I remember Paul mentioning that on a stream, but it doesn't seen viable or desirable. Assigning roles should be how you gain a tactical advantage. I think you shouldn't be able to assign the support role to a ship without a support module or the capital role to a ship with a small or tiny hull, but beyond that roles should be totally player controlled.

This was something I noticed, but was waiting for Beta 4 to mention, but since you brought it up I'll comment. From watching the battle in the stream it appeared that the weapon types fire separately, meaning a ship with beam, kinetic, and missile weapons can fire on 3 targets. If this is true, large ships will be be much more effective against smaller ships if they have a mix of weapons rather than all one type.

I'm pretty sure transports will be support

In the stream on 1/30, someone asked a question that needs more discussion.  It was about allowing ships to stay out of range of short range ships.  This is a very viable strategy that I have used in many other games where you build specialized ships to put in a "kiting" fleet.  these ships are designed to be very fast with long range weapons and only work if you kite or shadow the enemy ships, keeping out of their range but staying in your range by using your superior speed.

Maybe there needs to be a maneuver "kite" or "keep at range".

while i dont want direct controll over the fight if the general/commander in charge of the fight is going    "All ships full speed ahead and fire everything you have"  that general / commander is going to be a head shorter when i get through with him

While I agree that it would be good to see the ships act a little smarter, it is important also to avoid cheese tactics. Being able to load up a ship with thrusters and a single laser and auto win against anything with shorter range would be crappy.