What are you talking about? The "YF-12C" designation was not a combat aircraft. It was a fake designation slapped on a regular SR-71 that NASA used for flight testing in order to hide the secret origins of the program. There was never a real YF-12C.

You might be confusing the fake YF-12C with the real YF-12A. Both designs were derived from the original A-12 and look superficially similar but are internally quite different. There is no practical way to modify an SR-71 to carry and employ weapons. I mean it would be like trying to convert an F/A-18F into an F/A-18G: not technically impossible but so complex and expensive as to be utterly pointless. If the SR-71 ever actually flies again it will be used for nothing more than aerodynamic tests.