Kamiya's a pretty proud guy, and his integrity probably wouldn't let him mix his great work with the work of the same company that stole what was his in the first place and changed it.
The thing is, they never really "stole" anything from him, DMC was always owned by Capcom to begin with.
He was merely just a guy who worked for Capcom who happened to turn what was meant to be RE4 into a popular series.
He hasn't even spoken out against the series until now which obviously means he wasn't to against what Capcom was doing with DMC2,DMC3,and DMC4.
It wasn't until DmC was revealed that he started to show regret for not being in charge of the series.
And why wouldn't he use DMC3?
it IS the most popular entry to the series he created.
Kamiya really doesn't strike me as the guy to ignore something that could be used to make his work even better out of sheer pride, especially something that moved the series he created to a higher level.
You make it sound as if he is a headstrong guy who would poop on and disregard other games simply because he did not make them which isn't the case.
I'm sure he would be perfectly willing to take cues from DMC3 if he felt it would make a better DMC game.
After all, as I said, it is generally believed the DMC3 brought the series he created higher rather than lower.