As stated earlier, the only way to check a 3A3C HV rectifier tube is by substitution in a working set. Tube checkers throughout the ages have never been able to weed out any HV (1B3, 1J3, 2AV2, 1V2, 3A3, 3AT2, etc.) tube. Tube testers (by design) use a low voltage to test HV tubes. My Sencore from the 60's has a paragraph about them - (paraphrasing) "don't expect to see the filament light up, test is go/no-go for Anode to Cathode shorts only, use a working device to determine merit of tube"
Is the Shift Key on your computer broke?
Cheers,