I'm not really sure in what capacity Sybil will be working in S3. No matter what happened historically (ie woman getting fired once they were pregnant, low wages and long hours), JF will probably do whatever he wants to suit where he wants to take the characters. After all, there were plenty of historical inacurracies regarding Sybil's nursing in S2, so if he wants to have Sybil working while pregnant and with small children he will find a way to do it, regardless of whether or not it actually happened in 1920.

scarletcourt in regards to your list as to why Sybil married Branson and not some liberal minded aristo or upper middle class guy, I got the impression that part of it is b.c Sybil doesn't really believe in the class system in which she was raised. When Robert told Branson to address Sybil as "Lady Sybil" in ep 8, she called it "nonsense" and I remember hearing (though for the life of me I can't remember where) that JBF once commented that she thinks Sybil feels almost "embarrassed" having been raised in the lifestyle that she was.