每3年1次用英语怎么说？？

once every three years
或者one time every three years
once 相当于one time
三年两次就说 twice every three years

one time (once) every three years
one time (once) each three years
one time per three years

直接是"every three years"就行了
eg.The competition is held every three years.
这项比赛每三年举行一次.

once every three years

once every three years

one time every three years