I can't figure out what i did wrong. I have Vatras and Pyrokar waiting at the stone circle, have 3 swampweed and eye of innos with repaired setting.

But Xardas won't leave his tower, I just get the END options for dialog. He did say once that "I'm not going there until you're strong enough to fight the dragons, and you're not strong enough" I've completed almost every side quest, can't figure out what I missed. My strength is over 200, so I'm not sure what he means.

I'm about to cheat and resort to giving myself the charged eye of innos & start chapter 4 manually (how?) but I'd rather do it the right way and watch the fireworks at the stone circle.

Have you tried just returning to the Stone Circle? If you've already got Vatras and Pyrokar to go there, I think Xardas will show up on his own.

I don't think it has anything to do with your actual Strength score.

Sorry if this isn't helpful. I don't think I've heard of this exact problem before.

Are you still militia, mercenary or novice?
I think you have to advance to a higher rank so you must become paladin/dragon slayer or mage and you will also get new armor.

I'm militia - That did it, I forgot to talk to Lord Hagen about serving the order. Once i did that, I had the option "I'm ready to fight the dragons" and he made his way to the stone circle.

Thanks!