Sagamore to Cambridge via Plymouth, Weymouth,, Braintree, Quincy and Boston
(Yes, the map shows both US 3 and MA 3. The changeover occurs in Boston.)
Limited access highway from Sagamore to Boston.
Originally NE-6. In 1930, MA 3 used current MA 3A to get from Namelot Heights to Kingston. It then took modern MA 53 from Kingston and up into Quincy. It took the Southern Artery to Neponset, the followed over to Morton St via current MA 203. The map is unclear; it appears that it then followed the Jamaicaway north, crossed the Charles and becomes US 3.
By 1956, the highway for MA 3 existed from Sagamore to Kingston. MA 3A took over the coastal routing. Between 1961 and 1966 (probably closer to 1961 than 1966) the current MA 3 highway was completed and signed as 3. Northern portions of the highway were signed as part of MA 128 until re-assigned to MA 3.  53 was created to take the old routing. (3A was unavailable, as it was the coastal routing from its creation).
In 1971, MA 3 was taken off of its surface-street routing and put ont othe Southeast Expressway (now I-93) between Quincy and Boston. (MA 203 was created for the section between the expressway and Jamaicaway). Once in Boston, 3 got off the highway to Charles St and headed for Leverett Circle. My 1971-72 map shows a 3N going over the Longfellow Bridge and a 35 crossing via Charles River Dam and Commercial Ave. (This may be a typo, more likely it is 3S). MA 3 then extended down Memorial Drive to the changeover to US 3. Official state maps of the 1970’s show this change at the Harvard Bridge; AAA shows it as the BU Bridge. When the Longfellow Bridge was opened to 2-way traffic, Rt 3 was put onto that and taken of the Charles River Dam/Commercial St combination.
 As pointed out by John Carr in a personal email, the speed limit regulations for Rt 128 in 1960 included a stretch from Quincy to Weymouth.