Well, yes, it is a limitation of the radios, but Chirp only added the duplex "off" feature recently. Radios that were added to Chirp before this feature was added won't have it until a dev goes back and updates it.
That's why I said "these radios." All the chinese clones that I have seen store independent TX/RX frequencies and go TX inhibit when you zero the TX side.
If it's just a matter of a few minutes of update,
It's also a matter of getting someone with the radio to save two channels, one with TX inhibit and one without and no other differences. Then they create a bug report and post their img. Describe where to find the two channels and a Chirp dev should be able to look and the img and locate the TX inhibit bit.
So, for this radio, I think it's doable without a sample image.
How about this. Benton, if you will file an issue for this on the website:
http://chirp.danplanet.com/projects/chirp/wiki/How_To_Report_Issues
and attach an image of your UV-3R, I will quickly make the change to one of the channels, let you confirm that it inhibits transmit, and then I'll fix the code to make it available to the user, okay? No pizza required :)
--Dan