I think it's this. As a poly person and someone who is a part of a throuple (my husband and I are involved with the same woman), I've been aware of this word for some time, but I've also noticed it more in the news lately. For example, the three men in California who recently won the right to all be listed as parents on their child's birth certificate, which you mentioned but I've also seen several other news stories about throuples as well, so I do believe it's becoming a more common relationship configuration.

The rise of polyamorous parenting: Why 'throuple' families are coming out