According to Benedict Cumberbatch, filming for Doctor Strange in the Multiverse of Madness was previously reported to begin in early-November. This does make everything seem quite tight, considering he will be reprising the role of Doctor Strange in Spider-Man 3 while Elizabeth Olsen was still finishing with WandaVision.
There have been some questions about whether filming for the multiverse movie was delayed slightly from when Cumberbatch was last informed. Olsen has already revealed that filming for Doctor Strange in the Multiverse of Madness would last until at least Christmas for her, but according to the latest Empire interview, Olsen is apparently already filming the Doctor Strange sequel in London.
In the expose on WandaVison in the latest issue of Empire, it was noted that by the time the issue was released, Elizabeth Olsen would be in London shooting Doctor Strange in the Multiverse of Madness.
Elizabeth Olsen actually specifies that she will be filming for Wanda's character "back-to-back," implying that she began filming for the Doctor Strange sequel shortly after filming wrapped for WandaVision.
"I've spent the last year with Wanda. And it's actually incredible to go back-to-back because I feel I can contribute so much more now."
WHAT THIS MEANS
Elizabeth Olsen's filming schedule actually tells fans quite a bit about other productions since filming for WandaVision apparently reached an end around the beginning of November. At around the same time, Benedict Cumberbatch was on the set of Spider-Man 3 in Atlanta, Georgia, by at least the end of October.
If Elizabeth Olsen did indeed begin filming for Doctor Strange in the Multiverse of Madness shortly after finishing with WandaVision, that could imply that Benedict Cumberbatch didn't shoot for long in Atlanta for Spider-Man 3. At a minimum, Cumberbatch could have been shooting in Atlanta for two weeks, maybe three.
Still, just because Olsen has started filming doesn't automatically mean that Cumberbatch must be present too, as there are likely scenes with Wanda without the Sorcerer Supreme. Olsen and the team could film such scenes first before Cumberbatch arrives.
It is still unknown what kind of role Doctor Strange will have in Spider-Man 3. Still, considering it seems more and more likely that a multiverse element will be present in the movie, his role has to be more important than a small cameo like in Thor: Ragnarok.