Official Sonic 3 theater count from Paramount is 3,800 theaters with previews starting at 3pm, though I personally don't see any before 5pm Dec 17, 17:20
The 3pm showings are listed separately at AMC as "Fan Event" Dec 17, 18:10
Ah, that explains it. Didn't come up in a Google search {nm} Dec 17, 18:42