I am also interested in this adapter but I guess this post was overlooked. So I have the same question, is this adapter supported?
I found this post Support for RTL88x2BU WiFi driver - #2 by afl1 but it only mentions the RTL8822BU driver and according to https://wikidevi.com/wiki/TP-LINK_Archer_T3U this dongle actually uses the RTL8812BU.
It would be nice to have a comprehensive list of drivers/chipsets supported by coreelec (which gets extended when new ones are added) and honestly that was what I expected when I opened this thread. So maybe it is possible to extend the first post with such a list.