Campain 3th mission

Actually, that's campaign mission #4 and I'm in the same boat as you! Boy do I need help! I'm not sure what I can do to help not getting creamed by the Dread lords. I even lowered the difficulty to "nromal" and still got beat... beat bad.

Don't try to fight the Dread Lords in this mission; just complete the mission objectives i.e. Take over the Drengi planet.

Ok, heres the strategy I use. It needs to be intelligent, so your allies can work on tech with you. Figure out what they're researching, and do something else, share out the tech. The more planets the better, more research and the Dread Lords get distracted. You need plasma weapons to beat down the Yor, but thats later. First, you need medium ships to have a hope against the DL. Then, soldiering tech. With soldiering (planetary invasion, defence, and tir quan) all maxed out (maybe even soldiering and courage bonus) the DL won't be able to invade you anymore, unless they bum rush your planets, so your ships need to be fast. So the second thing is warp drive.

Held them off? good, this took me about ten tries before I tried it this way. Get those plasma weapons, you should still have 4 or 5 planets. Recover some from the DL if you can (You'll win with 5000 troops easy by now). Get your fast, powerful warp+plasma ships near that damn Yor planet, and clear the way. Get an escort fleet to protect transports from the DL. Take the planet, then charge head first to the objective. You should be ok, 3 transports per Yor/Drengin planet with all your bonuses.

This tactic took me two tries, but it generally works well. If you're having problems early on, get a big sensor ship to see when transports are on their way, use those fast ships to dodge fighters and take them out.

You odn't need medium ships. Tiny fighters with 2 laser cannons and a high logistics ability will let you make fleets of small ships that can effectively damage the Dread Lords. However, on this mission I'd advise you to expand fast, use fast cargo ships with a laser to take out DL transports, and take over that planet ASAP, i.e. before the DL's wipe you.

Cheat

Also, you can simply give up. There's an alternative mission, which is a lot easier

Just like someone said earlier...focus on the mission

Ignore the DL. Ignore the Yor.

Pump out long range transports with a light fighter escort that can reach the Dregni planet (starbases help here getting the range you need). Create several such fast & light task forces to converge on Dregni planet simultaneously from different approaches. Chances are that one fleet will get wiped by either Yor or DL, but your remaining forces can take the objective.

This can be done before the DL's get too frisky.

I completed the siege mission with the following tactic.

I did not colonize any planets just made a trail of star bases so I could reach the Drengi planet. Quickly wiped out the defences and sended in the troops. It´s tough mission, just don´t start to fight DL.