They were noticeably absent from today's press release from Marvel Studios that unveiled the main cast and characters returning or being introduced in Thor: Ragnarok, but you should still expect to see Lady Sif and the Warriors Three together again next year.
The four supporting characters played by Jaimie Alexander, Zachary Levi (Fandral), Ray Stevenson (Volstagg), and Tadanobu Asano (Hogun), respectively, are the most loyal of friends and battlefield allies to Thor. And when prodded about the absence of Sif in the Thor 3 cast announcement, Alexander immediately told her fans, "don't worry." So don't.
It's odd that if these characters were to return that they'd not be mentioned by Marvel but it's not uncommon in Marvel press releases where for every single movie (including Doctor Strange most recently), certain supporting characters were confirmed later (in that case, Benedict Wong). It's possible they're being saved for another, separate announcement alongside more concept/character art. Below is a key frame from Thor: Ragnarok that coupled today's news for instance - news that focused on the main returning characters, but even moreso on the four key additions (Cate Blanchett, Jeff Goldblum, Tessa Thompson, and Karl Urban). Details at the bottom on their roles.
Last year, Zachary Levi, who too over the role of Fandral for the sequel, said he hadn't heard yet if he would reprise the role for Thor: Ragnarok but was very excited at the possibility. And Ray Stevenson, who played Volstagg, said he heard that "it's on the cards," but nothing official since. It's Alexander's words today that lend the most credence to the idea that these supporting players will be returning (something we feel is obvious):
Don't worry ;) .... https://t.co/LvbYNWmBNa— Jaimie Alexander (@JaimieAlexander) May 20, 2016
There's also the little fact that "Ragnarok" means "the end of all things" (the initial pitch for the movie was "too dark" - a big reason Taika Waititi was brought it to helm the feature and lighten it up) so Sif and the Warriors Three are a must when it comes to the epic (and potentially final) battle for Asgard. Having them all return to round out the Thor trilogy also gives Marvel a chance to deliver some high stakes casualties to push the characters along - as hinted at by co-writer Christopher Yost.
Ragnarok is the end of all things. What are we supposed to do, let people live?!— Christopher Yost (@yost) November 6, 2014
Then again, even if Asgard goes 'boom' and Ragnarok is what it is in the comics, the Nine Realms will come back anew, albeit different, but don't assume Thor: Ragnarok is the end of the Thor series. Outside of the the Avengers flagship franchise no Marvel Studios movie series has yet to have a fourth movie announced but we're only three phases in. Iron Man 4 should have been the first and it could still happen in Phase 4 of the MCU (beginning in 2019-20) given star Robert Downey Jr.'s renewed interest in the MCU and role of Tony Stark. He's re-optioned additional pictures to his contract multiple times, most recently to co-star in Captain America: Civil War and again to play a key supporting role in next summer's Spider-Man: Homecoming.
All that is to say, Iron Man 3 and Captain America 3 are billion dollar hits in an ongoing concern. Obviously they're going to get followups, and Thor 3 is clearly being built up to accomplish similar (hence Hulk co-starring) box office milestones. With it featuring a pair of original Avengers alongside fan-fave returning faces and four new incredibly talented and geek-friendly actors, Thor: Ragnarok can hit that billion dollar mark too and it's going to need to include everyone like Civil War did.
And if it does, and even if Chris Hemsworth chooses not to re-sign for more movies post Avengers: Infinity War, Thor 4 can star another character. Thor's currently a woman (Jane Foster, who's not returning for Thor 3) in Marvel Comics now and Ragnarok is introducing Valkyrie (Tessa Thompson) who could one day carry a movie. Valkyrie did serve among the Avengers in the comics.
The point is: these aren't ever "solo" movies anyway in the truest sense of the word. Especially not now when every film is a team-up. Where RDJ's Tony Stark carried much of the screen time in Iron Man by himself, Thor (like Captain America) is a leader - that's one of his talents - and at his side are Loki (for better or worse), Lady Sif, and the Warriors Three. And if Thor and Hulk are off on a cosmic buddy adventure with Valkyrie, who's left to protect Asgard?
More: 11 Things You Need To Know About Ragnarok
Two-time Oscar®-winner Cate Blanchett (“Blue Jasmine,” “Carol, “Cinderella”) joins Marvel Studios’ “Thor: Ragnarok” as the mysterious and powerful new villain Hela, along with Jeff Goldblum (the upcoming “Independence Day: Resurgence,” “The Grand Budapest Hotel,” “Jurassic Park”) who joins the cast as the eccentric Grandmaster, Tessa Thompson (“Creed,” “Selma”) who will bring the classic hero Valkyrie to life on the big screen, and Karl Urban (“Star Trek” trilogy, “The Lord of the Rings: Return of the King”) who will add his might to the fray as Skurge.
Rounding out the cast for the film is three-time Oscar®-nominee Mark Ruffalo (“Spotlight,” “The Kids Are All Right,” “Foxcatcher”), reprising his role of Bruce Banner/the Hulk from “Marvel’s The Avengers” and Marvel’s “Avengers: Age of Ultron.”
Thor’s new cast members join returning stars Chris Hemsworth (“Rush,” “In the Heart of the Sea,” “Ghostbusters”) in the title role for the fifth time; Tom Hiddleston (“Crimson Peak,” “The Night Manager,” “Kong: Skull Island”) as Thor’s adversarial, adopted brother, Loki; Golden Globe® and Screen Actors Guild Award winner Idris Elba (“The Dark Tower,” “Luther,” “Beasts of No Nation”) as the Asgardian sentry, Heimdall; and Academy Award®-winner Sir Anthony Hopkins (“Silence of the Lambs,” “Nixon,” “Hitchcock”) again portraying Odin, Ruler of Asgard.
Source: Jaimie Alexander