The Sisters of Mercy are moving out of St Michael's Convent, Uckfield, next month following the sale of the building.
Sister Lucy, Sister Barbara and Sister Anastasia are due to move to Midhurst, where their order is based, on November 2.
A Mass of Thanksgiving is to be held on Sunday, November 1, at Uckfield Catholic Church for work done by the Sisters of Mercy in the town over more than 100 years and particularly for the work done by the last three sisters.
A special fund is being set up for them and it is hoped to make arrangements for them to make regular visits back to the town for special events at the church.
Sister Anastasia arrived in Uckfield in 1956 and a year later helped dig the foundations of the new Catholic Church in New Town.
She was matron of St Michael's boarding school at the convent and also taught biology and some RE.
The boarding school was closed in 1987 and Sr Anastasia helped move a nursery school, which has since closed, into the convent.
St Lucy arrived in Uckfield in 1966. After a year teaching she became head at St Philip's School, a role she held for 17 years.
Following retirement she was a governor for five years. Then she was asked to go to Hill House prep school where she stayed until 1993 when the sisters relinquished charge of the school which re-located to Mayfield.
Sr Barbara came to Uckfield in 1970 and was teacher in charge of the nursery which moved into St Michael's Convent.
• Plans have been approved to demolish St Michael's Convent and build 30 flats in two blocks and a 25-place nursery school on the site.