单次的英文怎么写

once 第一.twice 第二，third 第三，fourth 第四，fifth 第五，sixth 第六，seventh 第七，eighth第八 ，ninth第九，tenth 第十。

once of three years

一年一次 once a year一年三次 three times a year三年一次 once every three years

every three yearsxxx is held only every three years.xxx is held once every three years.

一次：once两次：twice三次：three times四次：four times五次：five times六次：six times七次：seven times八次：eight times九次：nine times十次：ten times