Did you buy either Scrivener 2 or Scrivener 3 from the Mac App Store by any chance? If so, because of the way the Mac App Store's sandboxing partitions things, Scrivener 3 will not be able to see Scrivener 2's project templates. However, if you bought both from our own store, Scrivener 2's project templates should still show up.
If you bought Scrivener 2 from our store, the templates will be at this location:
(You can access the ~/Library folder by holding down Option/Alt when opening the "Go" menu in the Finder - this makes the "Library" item visible, which is normally hidden.)
You can then copy the templates to a different location temporarily, and then use the "Options" button in the project templates chooser in Scrivener 3 to access the "Import Templates..." option, and choose the files you placed in the temporary location to import them.
All the best, Keith
"You can't waltz in here, use my toaster, and start spouting universal truths without qualification."