WASHINGTON — Looking for a way to persuade more women to enlist in the Navy and the Marines, the two service branches have expanded a benefit, tripling the amount of time off that new mothers will be given after the birth of a child. Under a policy change announced Thursday, female sailors and Marines will get 18 weeks of maternity leave, with pay. That’s far more generous than the standard set by the Family and Medical Leave Act, which requires that women be allowed 12 weeks off and does not require that they be paid.