The 3,000 Russian solders killed seems about right, although I'd like to see the original source if you could provide it. However, the official estimates of Ukrainian citizens killed, both on the separatists side as well as the army, seem unrealistically low. Then again, I can't see how it'd benefit the current Ukrainian government if those numbers were much higher. 

On the other hand, even Putin admitted that there are many Russian citizens in eastern Ukraine. Many of them are fighters. I am certain there are plenty of Russian intelligence officers there. Also, on 2 or 3 occasions Russian military did cross the border en mass, bailed out the separatists and promptly crossed back into Russia. And I'll even add that most likely there is a strong connection directly between the Russian military and the insurgency leadership.

But all of that, even if proven true beyond any doubt, would not constitute an invasion. It's kinda like calling a pat on the shoulder an aggravated assault.