Passed along by Andrew (thanks!).
This is from 2005 - I assume that there have been more of these types of marriages since.
From the Brussels Journal:
First Trio "Married" in The Netherlands
The Netherlands and Belgium were the first countries to give full marriage rights to homosexuals. In the United States some politicians propose “civil unions” that give homosexual couples the full benefits and responsibilities of marriage. These civil unions differ from marriage only in name.
Meanwhile in the Netherlands polygamy has been legalised in all but name. Last Friday the first civil union of three partners was registered. 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.
“I love both Bianca and Mirjam, so I am marrying them both,” Victor said. He had previously been married to Bianca. Two and a half years ago they met Mirjam Geven through an internet chatbox. Eight weeks later Mirjam deserted her husband and came to live with Victor and Bianca. After Mirjam’s divorce the threesome decided to marry.
Victor: “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.”
Asked by journalists to tell the secret of their peculiar relationship, Victor explained that there is no jealousy between them. “But this is because Mirjam and Bianca are bisexual. I think that with two heterosexual women it would be more difficult.” Victor stressed, however, that he is “a one hundred per cent heterosexual” and that a fourth person will not be allowed into the “marriage.” They want to take their marriage obligations seriously: “to be honest and open with each other and not philander.”