Adding new partners

I don't have experience with this, though I have had a V turn into a triad. It happened in a very natural, organic way.

How well do you guys know this person? Does this person know one or two of you better than the others? Is this person interested in something casual or more invested relationships? Does this person have any poly experience? Knowing nothing about this person, and whether they might have weird expectations of poly, makes it difficult to advise you.

I think that it's important to evaluate this person's motivations - not to say that they have bad motivations, but I could imagine a situation where someone learns of poly through you three, gets really excited by the fact that poly exists, and decides to pursue all three of you. Such a situation would maybe not be the ideal way to start a relationship between all three of you and this person. Although it isn't an insurmountable hurdle, it just would require communication to get everybody on the same page. If this person is going in with assumptions that are incorrect, it will help to address those things sooner than later.

If I were you I'd try to let things develop organically. Hang out with this person, all as a group as well as individually. I would start out with that in situations where sex is not on the table (public places) unless a casual sex relationship is the goal. Make sure this person understands that he/she doesn't need to develop romantic relationships with all of you, and that the relationships that do develop do not need to be equal in depth, level of attachment, amount of sex, etc. and that it would be highly unlikely for those things to develop equally.

I'm speaking from my perspective, and my preference is moreso one of developing loving relationships. If its casual then it might be less complicated.