Tarantino didn't "plan" for a 3rd movie, rumors have just existed about it. In case you haven't noticed he's not really the type to do sequels. He didn't even want it to be 2, it was supposed to be one movie but the studio made him split it so it wasn't 4 hours.
Why are the films two very distinct and separate styles then? Genuine question.
Well it wouldn't exactly be a stretch for Tarantino to have a stylistic change part way through the movie, especially if it was a 4 hour movie. "The Whole Bloody Affair" cut is supposed to be more what he originally wanted, there's a bunch of small changes that I forget, there's more violence, extended fight scenes, extended anime scene, The Crazy 88 fight is is color, and more significantly the reveal at the end of Vol. 1 that her daughter is alive is cut entirely so she doesn't find that out until the end of what would be Vol. 2 when she's standing in front of her. Which personally I enjoy the ending scene a bit more taking it from that perspective.
It would be a massive stylistic change though. They're completely different genres of film.