not to mention let alone needless to say 感觉前两种用的更多.这三个短语基本上通用的,但是let alone更加书面一点,其它两个比较口语化,语法上没有区别的. One company is needed to take care of the gangsters,not to mention/let alone/needless to say a regiment
只需要一个连就能消灭这帮顽匪,更不用说一个团了。译为:Needs 1 to connect and then can exterminate this to help Wan bandit and said nothing of a regiment. “更不必说”译为Go without saying more...