Since the all three kings are tanks, you should use the crystal tower against them. I seriously had a hard time with this level, even more than the final one. When I played through with limited resources I kinda knew how to do it, so I only had to retry like 3 times. What I do is this:
I try and kill the first king as fast as possible, because if all three kings send out waves of enemies, then you will have a really hard time. Once the first king is dead (usually works sending rouges and those swarmer soldiers) - I build a maze out of crystal towers (only these towers) so that the king has to walk a really long path to get to my base and encounter a lot of towers, I wish I had a screenshot to show you but I don't. Just try and build a big maze with as many crystal towers as possible. Once you manage to kill the first king, then the other two should be fairly easy, just keep upgrading and build a bigger maze as you go on.