Cambridge to NH line at Tyngsboro via Arlington, Burlington and Lowell.
(Yes, the map shows both US 3 and MA 3)
US 3 started on the Cambridge side of the Chalres River. It appears it might have come up Mass Ave to Arlington, then headed out on Mystic St and Cambridge St; as it does now. It follwed the same route as it currently has up to the highway; north of there it took what is now MA 3A up to Lowell. I’m not completely sure how it entered Lowell; quite possibly (and logically) on 3A all the way up to the NH border. Between 1942 and 1946, Lowell was bypassed and 3A took over the former US 3 in Lowell. The highway section of US 3 through Billerica and Chelmsford was scheduled to be completed in November, 1955. 3A was extended to cover all of former US 3. The highway north of Lowell was completed between 1967 and 1969.