In the 70's, American writers began looking across the pond to England for new ideas, or at least what they could adapt. For example, Steptoe & Son was rebooted here as Sanford & Son, using a mostly African-American cast. 'Til Death Do Us Part begat All in The Family. The racy sitcom, Three's Company, was an adaptation of Man About The House.
The basic plot is the same. Two young women, working for the same employer in this series, take in a male roommate to replace a female friend who has moved away. In order to get past the landlord's restrictions regarding relationships, the male, Robin Tripp, is passed off as gay, but by the 2nd episode, the landlord's wife finds out the truth, and, to get back at her husband, flirts with Robin.
When House was brought stateside as Three's Company in 1977, Robin Tripp became Jack Tripper. George & Mildred Roper were now Stanley & Helen Roper. Maintaining the links to its British origins, Company spun The Ropers into their own series, just like George & Mildred. Can't say for sure if Robin's Nest, the follow-up to House, is directly linked to Three's a Crowd, though.
From series 5 comes "Somebody Out There Likes Me". As you'll see, actress Sally Thomsett (Chrissy) was expecting, hence the maternity dress.
I think part of the reason The Ropers bombed was because Norman Fell, long a supporting player, had previously proven unable to carry a show as a lead, having previously top-lined Needles & Pins. Not sure about George & Mildred, though. House had ended production by the time it aired on WOR on this date in 1976.