a three-month extension的用法和样例：
- But we have a three month's probation at first.
- He was hired for a three month period on a trial basis.
- He is hired for a three months period on a trial basis.
- We will accept you for a three months' period of probation.
- The Reserves' skipper made five starts for Peterborough in a three month spell last season, and will provide further defensive cover for Paul Sturrock's injury-hit side.