The union is not a formal marriage because under Brazilian law that would be bigamy. Neither are they automatically allowed to declare joint income or join a healthcare plan for spouses.
But the civil union is still a big step, according to the lawyer.
"If they seek these rights before a court, they could obtain them - and I think they will," Leitao said.
They have a better chance now of making good on their plan to create a three-parent family, Leitao said.
"Our union is the fruit of love," the unnamed businesswoman in the trio told O Globo.
"We are preparing for my pregnancy. The legalisation is a way for the baby and for us to not end up abandoned and penniless.
"We want to enjoy the same maternal rights that everyone else has."
These are the first women to enter a three-way civil union, but a similar ceremony was held in 2012 for a man and two women in Sao Paulo state.
Both arrangements are based on a Supreme Court ruling that in 2011 authorised notary publics to hold civil union ceremonies for gay and lesbian couples, which the lawyer said can be applied to polyamorous relationships as well.
- AAP