I performed this successfully on my '17 A3 as well. Some things to keep in mind:
1. This only works on certain versions of Harman MMI hardware and, as pointed out above, only if your USB connections have data (if you can play music via wired USB to your unit in addition to Bluetooth, then you should be good).
For completeness, the back of the MMI will look as follows (the green roundish connector toward the left is the USB data+power going to the front and center consoles):
The above is true for certain model Audis which did not have the CarPlay feature enabled from the factory but otherwise had all of the requisite HW and SW. Some forums indicate that it was the early model year builds of particular model years that had this issue. It's not the same year for all models, but it was early build '17s for the A3 in the US (i.e. 2016 build date). As far as I can tell, dealers have no clue on how to do this officially because it seems that you should be able to purchase a "feature pack" of sorts from Audi that enables this officially. Anyway, I couldn't, so I went with MIB...
2. Check out the M.I.B. link:
https://github.com/Mr-MIBonk/M.I.B._More-Incredible-Bash. You want to read this a few times to familiarize yourself with the procedure. The trick is that once you are able to find everything on mibsolution.one, the patches only work on specific, official MMI SW/FW build trains. So, if your MMI isn't on one of their supported builds, you will first have to upgrade your MMI build via the same SD procedure, which has more caveats and requirements like disconnecting the B&O amp link. I didn't have to do this part, and this is documented well enough on other forums if you look. So, the takeaway is that you might have more steps to go through:
1) MMI SW update
2) Install MIB/Patch to enable the green engineering menu (GEM)
3) Invoke GEM/MIB to apply the "fixes" that enable the fun stuff
3. When you run any of it, wait for all steps to complete. The screen control is asynchronous and doesn't necessarily scroll with progress (at least mine didn't). But you can freely move the MMI screen up and down to watch for screen updates and only move to the next step once the current step is clearly complete. Luckily, it is smart enough to catch you doing the wrong thing (like it did with me, due to my impatience). The process may take up to an hour start to finish.
4. 4GB is the smallest size SD you need. I formatted mine to FAT32/4KB per instructions but may not be strictly necessary. I also used a full size SD card though I thought I saw them use a micro with an adapter in one of their videos.
Combined with a wireless CarPlay adapter, it performs nearly flawlessly! It's well worth the effort to undo Audi's silliness.
Good luck!