%triweekly%

a. [ Pref. tri- + weekly. ] Occurring or appearing three times a week; thriceweekly; as, a triweekly newspaper. -- adv. Three times a week. -- n. A triweekly publication. [ 1913 Webster ]

☞ 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. Cf. Biweekly. [ 1913 Webster ]