When I used to entertain fantasies of being married, I thought about this a lot. I went on a date to a wedding once, and we met the minister after, at the reception. She had only met the couple the night before, and was showing us a book she had of different ceremonies (it was a beautiful leather notebook). It made me feel very sad. I realized that I would never want to be married by someone who didn't know me, and likely they would have to know me pretty well.
Since your commitment isn't going to be legally recognized, and not likely recognized by any religious institution, you could have anyone do it. Do the three of you have some dear friend? If you were considering having someone be a best man or maid of honor, perhaps it would be better to have them officiate. It's easy enough to be ordained by the Universal Life Church (I am. I am also ordained by the Church of the Flying Spaghetti Monster (parmesan be upon him) which I absolutely love, and I think would be perfect for a poly commitment). If ordination is the sort of thing that's important to you.
You could search for wedding ceremonies on-line, and you could even conduct your own. You could have each one, in turn, read whatever, and make your promises to each other. When my mom and step-dad got married, they had the congregation affirm their support for the union, in a sort of call and response. I absolutely adored that, and should I ever, I want to incorporate that.
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who lives in the apartment building next door)