Word Study
triweekly
CIDE DICTIONARY
triweekly, a. [Pref. tri- + weekly.].
Occurring or appearing three times a week; thriceweekly; as, a triweekly newspaper. [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]
For further exploring for "triweekly" in Webster Dictionary Online