I doubt NATO needs to start WW3 if Russia used an NBC weapon, Russian doctrine is only to use its nuclear deterrent if its existence is under threat which effectively means if Russia itself is invaded and NATO is not going to do that however NATO air strikes on Russian positions in Ukraine doesn't threaten Russia's existence.
Now while it would be funny, I am sad to say no matter how bad the situation gets you wont see T-34s on Ukrainian battlefields as they simply no longer have any significant quantities.