It's been three years since my sister got married.
Literal
Younger-sister [subject-が] married [after doing-してから] 3-years [goal-に] become.
~してから~になる ('it's been X time since doing Y') is the standard pattern for elapsed time. 結婚してから ('since getting married') + ３年になる ('becomes three years') = 'it's been three years.'