Just going to pile on to the success list here, I've upgraded my 16 Explorer Sport to Sync 3 and it is working perfectly, just like a 2017 model. Only thing was I bought the wrong screen originally.
There are two different mounting styles for screens, and your brackets will not fit onto your new screen if you get the wrong one. Explorer owners need a Sync 3 touchscreen with a part number ending in -SC, otherwise you may have to modify your brackets.
If you find a dealer willing to work with you, they can purchase the screen directly from United Radio, Ford's radio part distributor, but when I checked on this the part cost (without dealer markup) was $720!!! Just search Ebay and scrap yards. I got one from a junkyard in Florida that had a 2017 explorer XLT that had been totaled with just over 1k miles on it. I paid $300 shipped, and it had a tiny little scratch on the screen, but to save $420 I'll take that any day.
I got a Nav APIM from Ebay for about $600, and my local ford dealership ordered the USB port for $70. Said and done, I upgraded to Sync 3 for just under $1k.
At the advice of PlatinumExplorer I downloaded the Asbuilt for a similarly-equipped 2017 Sport and programmed my APIM with that. I already had a OBDLink Wifi unit that I used to program the car with. Proper VIN showed up in Sync upon installation, it seems to just happily take that info from the BCM.
If you're missing screws that attach the Screen to the APIM, see if you can find an extra bracket on Ebay with screws. Those Torx screws fit perfectly. Alternatively, take one of them off and head to Home Depot to find a similarly threaded screw.
All the Ford part numbers I used:
USB Hub: HU5Z-19A387-A
Screen: GB5T-18B955-SC
APIM: HB5T-14G370-FCD
PSA: Try your best to know the history of your part if you can (if you find it in a junk yard on a wreck, or purchase a brand new part, those are the best scenarios). The rising demand of parts for this from people who want to upgrade means higher value, which means it could drive up vehicle break ins and radio thefts. Near as I can tell, 4dtech sells all Factory-new parts (That's where I got the incorrect screen from originally, and it shipped with the Factory screen protector).