Link: Spero Forum Weblog | Netherlands Gives Nod To Polygamy: Trio "Weds".
A trio had their "civil union" legally recognized in the Netherlands. While legal marriage is not possible for those who practice polyamory or polygamy, civil unions are:

The Netherlands gave an approving nod toward polygamy last Friday when "the first civil union of three partners was registered," according to an article written by Paul Belien for The Brussels Journal Victor de Bruijn (46) from Roosendaal “married” both Bianca (31) and Mirjam (35) in a ceremony before a notary who duly registered their civil union," reported Belien, whose article noted that while a legal marriage between three people in the Netherlands at this stage cannot be performed, such a possibility is allowed among civil unions. “A marriage between three persons is not possible in the Netherlands, but a civil union is. We went to the notary in our marriage costume and exchanged rings. We consider this to be just an ordinary marriage,” said Victor, according to Belien.

The article has the facts of the case, as well as some editorializing which is a little bit more than by far on the clueless side when the author makes a snide remark about bestiality. Sheesh.

In any case, despite opponents in the US, this may be the tact used for polyfolks there as well.