In Mac Scrivener 3, you want the File -> Export -> As Scrivener 2 Project command.
To avoid this in the future, you can use Mac Scrivener 2.9 for projects shared with Windows Scrivener. It will accept your Scrivener 3 serial number and can be downloaded here:
literatureandlatte.com/legacy-download
iOS Scrivener can read either format, but can’t convert between the two.
The Win Scrivener 3 beta can also read the Mac Scrivener 3 format, but it is a beta.
Katherine