Webster's Revised Unabridged Dictionary · 1913

Triweekly

Triweekly , adjective

[Prefix tri- + weekly.]

Occurring or appearing three times a week; thriceweekly; as, a triweekly newspaper.
Three times a week.
A triweekly publication.

This is a convenient word, but is not legitimately formed. It should mean occurring once in three weeks, as triennial means once in three years. Compare Biweekly.