The largest municipalities in the district are Lowell, Lawrence, Haverhill, Methuen, Billerica (partial), Fitchburg, and Marlborough.

In the redistricting that occurred after the 2020 census, the boundaries of the third district only changed slightly from its previous iteration.

In Middlesex County: Ashland, Holliston, Hopkinton, Marlborough.

In Norfolk County: Franklin, Medway, Plainville, Wrentham.

In Worcester County: Auburn, Boylston, Clinton, Holden, Northborough, Paxton, Princeton, Rutland, Shrewsbury, Southborough, West Boylston, Westborough, Worcester.