Not sure this is going to clarify anything, but let me repeat myself:
There is no blockage in the horn.
There is no blockage in the third tuning slide.
There is no blockage when the third valve is depressed.
There is only blockage of all air when the the third valve is NOT depressed.
My guess is, the third valve somehow got misaligned or is strangely in backwards. Clearly therein lies the problem.
I am going to bring it to Jonathan Ashcraft, my brass repair guy, who will surely know how to resolve it. He is a great repair guy, a regular artist, and he can play very well too.
One final comment: I am sure thankful I have a Gettzen Eterna to use every day. Every time I pick it up and rapidly press all the valves and they respond like a thoroughbred race horse, that is the feeling I get