I don't really think any of the 3 were the first metal band, just the main influences behind the genre. I mean if they're metal bands, you could say the Jimi Hendrix Experience and The Who were, a times at least, metal bands, really, since a lot of their work had similar qualities (with Led Zep and Deep Purple the most). Not to mention The Kink's "You Really Got Me" from 1964, that was also an influence on the sound of metal music.
I think out of the 3 though, Sabbath is probably the closest to the metal music sound we know, but that doesn't make them the overall first, just one of the primary innovators of the sound of it.
Personally, if I really had to name one to be the very first 'metal' band, Steppenwolf get my vote. Their debut came out in 1968 and I think they even coin the phrase 'heavy metal' in "Born To Be Wild".
Judas Priest is by no stretch of the imagination the first heavy metal band. They weren't considered to be part of the 'NEW wave of British Heavy Metal' for nothing.
I'd consider Sabbath to be the first heavy metal BAND. Bands like the Beatles (Helter Skelter), or the Kinks ( You Really Got Me) released some pretty heavy tracks for that time, but I wouldn't say they're heavy meta bands. Blue Cheer could be a contender for the title as well though.
Sabbath, Priest and Purple definitely had the riffs, bass and drums for what could be considered the blueprints for metal as did Zeppelin and to some extent so did the Kinks - for me You Really Got Me is one of the first metal song prototypes (flame away) but it had everything; the solo, the breakdowns and even the howls of Ray Davies could be attributed to an early forerunner for peeps like Gillan, Halford, Dickinson, Osbourne and pretty much every singer with a broad range of notes that's come out of the woodwork since.
However if you want to see where more conventional metal came from i.e. harsh vocals - and I don't mean just cookie monster growls a la Cannibal Corpse & Deicide etc but everything from Metallica's James Hetfield to Celtic Frost's Thomas Gabriel Fischer than look no further than Motorhead or Venom. (Even though Lemmy considers Motorhead Rock 'n' Roll)
And he also said the Beatles are bigger than Jesus, but somehow I don't see this in todays society. After having listeined to 'Ticket To Ride' I still stand by my original statement that metal owes more to the Kinks 'You Really Got Me'.