work experience varies amongst the schools you are considering. if you want to go to a top tier school, the average is around 4-5 years. you must also consider why you want an MBA. if you are looking to shift industries 180 degrees, then you will want to consider going full time to the best program possible in the field you want to get into. for this, you will probably need more than 3 years experience.
however, if you want to use the MBA to make an enhancement to the field that you are currently in, consider going part time to a regional school. bottom line - check the average years of work experience for the schools you are considering.