three month/months

[Mother1]

three month/months
Which one is correct?

[Dozy]

The usual form is "three months":

"I have been practising for three months."
"Three months should be long enough."

This is just like any other plural ("three apples", "three cats", etc.). What may be confusing you is that the form "three-month" (which should properly be hyphenated) is used as a modifier before nouns:

"There was a three-month delay."

The same principle applies to other nouns:

"The meal consisted of four courses." / "I enjoyed a four-course meal."