Mission 3 is impossible to win, because:
You start with like 4 units
Within 2 minutes about 30 enemy tanks have rolled into your base, and they keep coming in.
There is no way you can counter them.
Now that COULD be intentional… I remember the origional warcraft demo from about 1996 in the final level of the demo it was about 2 of your fighters vs 100+ orcs
ok the above should be fixed now, i have tried to soften up mission 3 a little hopefully should be completable now.
http://www.myjavaserver.com/~kappa/4k/c&c4k.jnlp
Cool! You did a great job handling those bugs. The game does feel more balanced now. However, I haven’t found a way to beat level 3 still 
Have you played it and beaten it yourself?
What are beta testers for? :
ok after playing it a few times and not being able to complete it, i’ve made some changes to the gameplay and i can say the level is now completable (having done it myself ).
Turrets are now stronger and the attack wave has been slowed a little.
http://www.myjavaserver.com/~kappa/4k/c&c4k.jnlp
Well I managed to beat 3, and almost beat 4. I destroyed all the construction yards for the enemies on level 4, and then like… time stopped. I stopped earning money, units stopped attacking. I could still move, and my units would still turn when I told them to attack something, but they wouldn’t attack. I still consider that a victory 
I might have still won! Strange bug though.
Also, the way you’re handling timing, I presume with Thread.sleep() is bad. It makes my system clock run double time. I played your game up to level 3, then looked down at the time, then looked over at my desk clock, and they were 10 minutes different. No good