Ill. Admin. Code tit. 11, § 306.30
Trifecta wagering shall not be scheduled on a thoroughbred or standardbred race unless at least five betting interests are carded. In the event of a scratch, trifecta wagering on a thoroughbred or standardbred race in which four betting interests remain is permissible.
(Source: Amended at 39 Ill. Reg. 13710, effective October 5, 2015)