I found this ritual on a Usenet post, and altered it slightly. It was written with 1 man and 2 women in mind, both women being involved with the man. This could, however be used for any configuration of involvements, genders and orientations. I have changed "Groom, Bride 1 and Bride 2" to "Partner 1, 2 and 3". Just insert names and change the pronouns and pronouncements as appropriate for your group marriage configuration.

Participants and witnesses joined in a brief breathing exercise designed to focus attention on the ceremony, and creating sacred space.

PRIESTESS:
Today, we are here for 3 reasons. We are here to honor the relationship that [PARTNER 1] and [PARTNER 2] have had for ___ years. We are here to wish [PARTNER 2] and [PARTNER 1] and [PARTNER 3] well in their voyage of discovery, as they explore the new paths that their relationship will take. But most importantly, we are here, in the sight of the Gods and all these well wishers, to share the joy in the relationship that [PARTNER 3] has found with [PARTNERS 1 & 2].

[The TRIAD joins together in a circle in front of the PRIEST AND PRIESTESS.]

PRIEST:
Over time, and with each relationship comes wisdom in love. You are enriched by all the loves you have ever had, and you bring that wealth to this relationship. May your share the bounties you have gained, and continue to grow in that wealth.

[PARTNER 1 speaks first, remarking on the vast changes she and PARTNER 2 have undergone, and that she welcomes PARTNER 3 into their family.]

[PARTNER 2 speaks next, placing a ring on PARTNER 1's finger, speaking of the changing of their relationship through the years, and how the ceremony is a new beginning for the triad, as well as how he will always love PARTNER 1 no matter how much their family expands.]

[PARTNER 2 next places the other ring on PARTNER 3's finger, speaking to of new beginnings, and his unending love.]

[PARTNER 3 thanks PARTNER 1 trust and generosity, assuring PARTNER 1 that she will always take PARTNER 1's feelings and needs into consideration.]

PRIESTESS:
If we learn no more of in life, let it be this: The law of life is love unto all beings. Without love, life is nothing. Without love, death has no meaning. Love is before life, and after death. Therefore, in the sight, and with the blessings of everyone here, we join with you, in celebrating your expanded marriage. Let the party begin!

GUESTS:
Heard and witnessed!