The S-3 was similar to the S-4 except that it used the un-turbocharged version of the 539 engine and so developed only 660 horsepower. It also had a smaller conical stack with a circular base and smaller square-looking radiator shutter assemblies. Between 1951 and 1954 CN purchased 49 S-3 units from MLW and these were numbered 8450-8498. S-3s 8484-8494 had narrower cab side windows. They were fitted with AAR trucks and had the riveted car-body similar to the early S-4s. Between 1964 and 1966 15 of these units were converted to booster units (slugs) which were paired with S-13 switchers. In the mid 1960s most remaining units were fitted with Watchman heaters and many with three chime horns, presumably because the units were assigned to light switching duties in remote locations. The last powered unit was retired in 1975 and the last slug about 1997.