You are assuming, I think, that the first two wives were still alive (and not divorced) at the time of the third marriage, to Gawhar. The "received wisdom" is that this was during his time in Baghdad, but (1) they had no children then, and (2) when Baha'u'llah was exiled from Baghdad to Istanbul, his "third wife" did not accompany him, remaining instead with her brother. They were later rounded up by the authorities and sent to exile in Mosul. She went to Akka in 1870, seven years after Baha'u'llah's exile from Baghdad. Their one daughter is said to have been born in Akka.
Now as to whether his first or second wife had died or been divorced by 1870 :
Baha'u'llah's last child with his second wife, Fatimeh, was Badi'u'lláh, born in 1867 when Fatimeh was 39 years old. By that time, Abdu'l-Baha had emerged as Baha'u'llah's favourite and pre-eminent disciple. Fatimeh is said to have been very jealous of Abdu'l-Baha ( son of Assiyeh). I see two possibilities: (1) Baha'u'llah and Fatimeh still had marital relations in 1870, despite her jealousy of Abdu'l-Baha, but she had no more children because of her age, or (2) Baha'u'llah and Fatimeh no longer had marital relations, because of age or because her opposition to his favourite son was intolerable. In the latter case, was there a formal divorce? Would we know if there was? We have no documentary or eyewitness account of the marriage, so it is quite possible there was a divorce, which no-one mentioned.
There is also a letter by Abdu'l-Baha, quoted in the Diary of Habíb Mu’ayyad and translated by Ahang Rabbani. The pdf was online, but has apparently been moved or removed. On page 443 it reads
The problem is, Khadijeh Bagum died on 15 September 1882, after the revelation of the Kitab-e Aqdas. Fatimeh was still alive when Baha'u'llah died. But Abdu'l-Baha says Baha'u'llah's third marriage took place after the death of one of his first two wives.
If indeed the marriage took place in Palestine after the death of Khadijeh Bagum, and if Gawhar was pregnant or had her daughter with her when she arrived in Palestine, (two ifs), then it would appear that she had a child and no husband, for which there are three explanations (1) she was a widow, as David Hofman says (but I do not think he is reliable in such matters), (2) she slipped or (3) she was raped. I think the last of these is most likely, as the group who were sent to Mosul were subject to severe abuse, and they had no protectors there.
There's a lot of uncertainty here, but the least improbable scenario is that Baha'u'llah married Gawhar when she was -- and because she was -- an unmarried mother.