The very first 737s were to be laid out for a 3-man crew, then in about 1968 the FAA changed the regulations which allowed 2-man crews only under 80,000 lb gross weight. United, an early 737 user, changed the spec on their order and there was a pilot's union dispute about it. Someone else here may know how it was resolved.
The 727 was well established before this time and always used a 3 man crew.
The 80,000 lb limit also led to a special version of the BAC One-Eleven to get under the limit with a 2 man crew (and in those days they were all just men !)