Blooper reels are traditionally made as annual entertainment for the cast and crew of series TV (or theatrical features) at their Christmas party, and were never really inended to scape into the public domain. Then along came TV specials and series, such as "TV Bloopers and Practical Jokes", but "Star Trek" has rarely been featured on such shows.
Trek bloopers became a feature of the early Star Trek conventions, a special treat for fans when Gene Roddenberry himself was in attendance. However, rumour has it, one convention committee, entrusted be GR to "mind" the reels overnight, managed to create their own hasty copies of the Season One and Two bloopers so they'd always have a copy on hand. And thus, bootlegs of bootlegs of bootlegs were distributed, much to the alarm of the cast.
The fans even made their own blooper reel, which is often called the third reel, but it's a cannibalisation of previously released footage, a little new stuff someone located, and reconstructed into a bit of a coherent story. And there was an infamous Season Three bloopers bootleg LP record, released by Blue Pear, which is made up of vocal track tapes found in a Paramount trash can.
The actual Season Three blooper reel has remained elusive. GR was more careful about keeping it close to his chest at conventions - supposedly Leonard Nimo wrote a memo asking if the cast shouldn't be receiving royalties each time it played - and fans who were lucky enough to see the reel at one of Roddenberry's US university performances in the 70s usually remember a sequence with Commissioner Bele (comedian Frank Gorshin, aka The Riddler of 60s "Batman" fame) and Lokai chasing each other through the Enterprise corridors, until they finally crash into each other. And it's there, along with Mr Gorshin calling himself "Belly" by mistake. Also Yarnek the rock creature singing "Mammy", some erotic Vulcan navel flirting (follow Nimoy's finger!), and footage featuring a pre-TNG Diana Muldaur.
After the bloopers, there are some (rare) alternate TOS titles and "coming attractions" ads. Who knows how long this particular entry will stick around on Youtube? So... enjoy!