Lady Sif and the Warriors Three have been absent from the marketing for Thor: Ragnarok, but that doesn't necessarily mean the fan-favorite characters are missing in action in the Marvel Studios sequel.
Screen Rant reports that although a visit to the film's set in Queensland, Australia, didn't reveal the whereabouts of Jaimie Alexander’s Sif, Ray Stevenson’s Volstagg, Tadanobu Asano’s Hogun and Zachary Levi’s Fandral, producer Brad Winderbaum did indicate audiences will learn their fates, for good or bad:
It sounds like we’re not seeing Sif and the Warriors Three, but do we learn what happened to them?
“Um… That’s not necessarily the case. And yes.”
That's certainly open to interpretation, but it indicates the characters will be seen, or at least referenced, in the film. For what it's worth, the actors are listed in Thor: Ragnarok's IMDb page.
We know Thor: Ragnarok sees Asgard fall at the hands of Hela, the goddess of death. It's been hinted that some characters could be killed, and with Lady Sif and the Warriors Three essentially cast to the sidelines in Thor: The Dark World, sacrificing them could make sense. Of course, even if they do survive Hela's initial attack, it doesn't mean they will escape unscathed. When we last left Asgard in Thor: The Dark World, Odin had been secretly replaced by Loki. If the heroes were tricked by this, it's possible they were either imprisoned or exiled by Loki in order to ensure his rule.
Arriving Nov. 3, director Taika Waititi’s Thor: Ragnarok stars Chris Hemsworth as Thor, Tom Hiddleston as Loki, Cate Blanchett as Hela, Idris Elba as Heimdall, Jeff Goldblum as Grandmaster, Tessa Thompson as Valkyrie, Karl Urban as Skurge, Mark Ruffalo as Bruce Banner/the Hulk and Anthony Hopkins as Odin.