Congrats on your soon-to-be new Model 3.
I can’t answer specifically for a M3, since I have a MS as most people reading this thread likely do, since this thread is in the Model S section. You may want to create a post in the Model 3 section of the forum to get current M3 owners to weigh-in.
As it has been for a very long time, MS (and MX) Media Player have a lot of bugs that can cause issues even using folder view. Generally, FLAC works, and you'll see and be able to playback the tracks in the folders you create using that part of the interface, but proper album sequence of those tracks may be out-of-whack depending how your physical files are named. MP (sadly) does not use the DISC+TRACK tags to figure out the order of tracks (and discs in a multi-disc album) to playback -- but instead relies only the alphanumeric filename across all parts of the interface. There are sadly other possible issues related to album art, other ID3 tag anomolies, unexpected rescans, etc you may or may not encounter -- and not all of those are things you can always workaround. Take a look at the long-standing
Comprehensive USB Bug List thread for continued discussion as well as
the workarounds I use to build an MS-specific USB device to help mitigate many of the long-standing problems which then allow me to use the rest of the MP interface beyond folder-view. As MP has changed over time -- both improving and sometimes regressing -- I'd suggest starting at the end of that "comprehensive" thread and backing up a few pages, before reading forward to get caught up on what is going on with MS/MX USB if you are so inclined.
There have been many broken promises over the years, including a more recent tweet by Elon, and at least one article I read by the Press that an updated set of firmware is coming which will include some long-standing USB music fixes. As to if that code will be exclusive to the original 2GB MCU we have in our MS/MX, works with only the newer/faster MCUs being talked about in more recent builds, or perhaps benefit from any rewrites that may have taken place when creating the M3 Infotainment System -- we can only speculate at this point. I do know the latest firmware update my MS received late yesterday (2018.12) didn't address MP problems, only added the Tesla App ability to open the trunk and frunk according to the update notes.
Do take a look or post in a M3-specific forum to get input from the growing number of M3 owners there, because as time goes on there may or may not be differences in the underlying code and operation of USB MP. IDK.
Good luck with your new M3. Enjoy!