user: Question: What is the average First Elected, when District is "Massachusetts 3"?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Results']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Massachusetts 1', 'John Olver', 'Democratic', 1991.0, 'Re-elected']
SQL:

assistant: SELECT AVG("First elected") FROM "Table" WHERE "District" = 'Massachusetts 3';