I can't figure out why, but for some reason WoW will not recognize the updated version of TRP3. I've been using TRP3 since v1.2.x, and never had this issue before. I was running v1.4.6, and just updated to v1.5.2 this morning. After updating the files in my Addons folder I logged in, (I had the 'load out of date addons' clicked off'), and TRP3 didn't load upon entering the game. I logged out to my character selection screen, opened the addons window, and sure enough it read TRP3 as being out of date. I logged out of the game and tried extracting v1.5.2 to my WoW addons folder again. Still no joy. I logged out, deleted TRP3+TRP3Data from my WoW Addons folder. Then I deleted the Zip v1.4.6 file from my C:/... Downloads folder, so only the v1.5.2 file was left. Emptied the Recycle bin and restarted my computer, figuring I'd have to re-do all my character profiles now. On restarting, I extracted the v1.5.2 files into my WoW Addons folder, then started up the game. No word of a lie, it STILL reads as v1.4.6 upon entering the game, all my character profiles were intact, and when I opened TRP3 in game, even TRP3 gave me the pop-up window to tell me an updated version, v1.5.2, was available.
I have searched my C:/ drive, and dug through my WoW folders..... where is this info for v1.4.6 buried in and still coming from?

If this matters: I'm not currently using my regular pay-to-play account, I'm just using a free Trial Account. Also, I didn't try downloading the TRP3 extended version, only the regular TRP3 version.

The path to install addons changed with patch 8.1, it's now World of Warcraft\_retail_\Interface\AddOns. Make sure you're installing there and not in the old folder.

I thought that _retail_\ folder looked new. Yes, that was the issue. Thanks Solanya.