Semiweekly. Some people use biweekly, but this should be reserved for fortnightly, every two weeks. You should also be aware of the word "tertian" which usually means every second day but can also mean every third day.
Fortnight
Sesquifortnightly. Sesqui is a prefix meaning one and a half, like in the word sesquicentennial. A fortnight is two weeks. Every three weeks, therefore, is sesquifortnightly.
The spelling is one word, non-hyphenated, "biweekly" (every two weeks).
Every has two syllables.
Two.
bi weekly
A fortnight is a word used to indicate two weeks.
Fortnight
every other week or fortnightly
Bimonthly means every two weeks. One can tell by the word monthly which is included in the word bimonthly, giving this clear indication of what is intended.
Fortnight
A fortnight is a word used to indicate two weeks.
every two weeks
The base word for biweekly is week. The prefix is bi and the suffix is ly. With the prefix and suffix added the base word, it means twice a week or every two weeks, since bi means two.
Sesquifortnightly. Sesqui is a prefix meaning one and a half, like in the word sesquicentennial. A fortnight is two weeks. Every three weeks, therefore, is sesquifortnightly.
Bi-weekly can be used, but this can be confused sometimes as to mean 'every two weeks'. Semiweekly is another suggestion, but the intended meaning is still open to misundersanding
The spelling is one word, non-hyphenated, "biweekly" (every two weeks).