Spider-Man 3 wasn't part of Marvel Studios' Phase 4 announcement at San Diego Comic-Con, but it may still be added to the 2021 lineup. After a long wait, Marvel finally let fans know what is coming in the next Phase of the Marvel Cinematic Universe. The Phase 4 slate is only two years long but is actually quite ambitious. Marvel Studios confirmed Black Widow and Eternals for 2020 and then Shang Chi and the Legend of the Ten Rings, Doctor Strange in the Multiverse of Madness, and Thor: Love and Thunder for 2021 releases.
In addition to those five movies, Marvel Studios will also release five original series' for Disney+ and they have plans to do more movies down the road. But, amid all of the surprises that came during Marvel's SDCC panel, one movie that was noticeably absent from any of the discussions was Spider-Man 3. This wasn't totally unexpected since Sony still owns, produces, and distributes the Spider-Man films even though they are now part of the MCU. The Sony/Marvel arrangement means that Sony, not Marvel Studios, is the one who will announce the release date.
It may not have been confirmed as part of Phase 4 during the panel, but it does appear that Marvel Studios and Kevin Feige may have Spider-Man 3 in their plans for 2021. During a post-panel interview with MTV, Feige mentioned that they have eleven projects in the works for the next two years. While this could possibly be Feige including Blade among the five movies and five TV shows announced, it has been assumed that film will release as part of Marvel's 2022 slate and thus wouldn't count in the 2020/2021 discussion. As such, it could just as likely be Feige having a rare misstep and alluding to Spider-Man 3 being part of the 2021 slate.
As part of Sony and Marvel's deal, they've so far released Spider-Man: Homecoming in 2017 and Spider-Man: Far From Home just a few weeks ago. Both films hit theaters in July and have operated on an every-other-year release strategy by Sony. As of right now, they have not confirmed when the next installment will come, but 2021 makes sense. This line up with Marvel's confirmed 2021 releases in February, May, and November: there is a clear landing spot for Spider-Man 3 in July, which is why we've been predicting a 2021 release for the trilogy capper.
The question, really, is when this will be confirmed or denied. Sony announced Spider-Man: Far From Home would be released in 2019 months before Spider-Man: Homecoming hit theaters, so their strategy is already different with Spider-Man 3. Because of this, Sony could confirm Spider-Man 3 is coming in July 2021 at anytime. They could possibly be waiting until Spider-Man: Far From Home is out of theaters to make this announcement or could do so once it crosses the $1 billion mark worldwide. The latter gives them a chance to celebrate the milestone with a guarantee that more movies are on the way.
In any case, if there is an eleventh MCU Phase 4 project, Spider-Man 3 is the best bet. Just as how much the original Phase 3 slate changed, this Phase 4 slate isn't guaranteed to be all we'll get before 2022. If Spider-Man 3 does hit theaters in 2021 though, it will be the first time that four MCU movies have released in the same calendar year.