Something doesn't add up. As far as I understand (a) threads are external and (b) threads are internal. Are you planning on measuring internal threads with a ring? If i'm not misunderstanding you, I think there may be a typo on your print (has happened to me).
My Apologies I just read my thread.....Can I use a class 3A on a 2A Part? I have a part that calls out for a 1-1/2-12 UN 2A but all I have is a 3A. Since a 3A class is thighter tolerance...Can I use it on a 2a part?
You can use a 3A NO-GO ring gage, as a reference to establish conformity on the minimum side of the pitch diameter scale, but not on the maximum, GO, as you would exceed the maximum on the 2A by .0019 if using the 3A ring gage. Make sure if you do purchase a 2A set, that you purchase also your set plug as your standard to that ring gage set, and it needs to be 'W' tolerance. (ref. ASME B1.1 for 2A/3A ,limit size of standards)