R3 applicant here. Submitted evening of 3/15 and received invite to interview today. Status changed directly from "Received" to "Invited to Interview."
On a separate note, Courtney Chandler, one of the admissions directors, confirmed during an infosession that the reason they waitlist so many applicants is precisely to make R1, R2, and R3 equally competitive. With that in mind, it seems like it would make sense that many applicants are admitted in R3/R4 since many R1/R2 WL applicants and R3/R4 applicants are all admitted together.