Did Trump say “Cheat / Cheater / Cheating” at St. Clair County, Michigan?

Yes. Said at 3:30:42 PM ET, flagged live 0.64 seconds after the word.

“We gotta be careful with the cheating vote counters though that's one thing I gotta tell you”

From Saidwire’s live log of Trump rally · St. Clair County, Michigan. All 13 words said at this event.