There are several Kingdom Hearts 3 trailers that were shown off behind closed doors and were never released to the public.
The first and most interesting one was shown off at the Kingdom Hearts 2.5 HD Remix launch event on December 1, 2014. No footage has ever leaked of this trailer, but there are several detailed reports online of what the trailer contains.
Another that I am aware of is the trailer shown off at the KH Orchestra during E3 2018. This trailer isn't as lost or as interesting because most of its contents were shown in later trailers released during E3. There were also several live reactions uploaded that included the trailer audio but not the video.
I'm pretty sure there were a few more trailers that were never released publicly, but the 2014 trailer is definitely the most interesting. If im not mistaken, it was the last time the game was shown off before they decided to switch engines and start from scratch. If anyone has any video of their event that would be dope as hell!