user: Question: How many seats became avaliable in Massachusetts 3rd district?
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['New York 1st', 'Jonathan Havens (DR)', 'Died October 25, 1799', 'John Smith (DR)', 'February 27, 1800']
SQL:

assistant: SELECT COUNT("Vacator") FROM "Table" WHERE "District" = 'Massachusetts 3rd';