Those are really good questions. I sat my hubby down last night and we talked about it. we decided the best thing would be to tell her up front what we are looking for. We are also realistic of human nature and realize that it is very likely she may develop a preference to one of us and when that happens the best coarse of action would be to have a sit down and possibly renegotiate the rules.
As for reading between the lines what I meant was that given her association with the local poly crowd has she already guessed our intent? I suppose I should add that we (the 3 of us) made out a little bit at a 4th of July party. We ran in to her at a picnic a few days latter and that is when we invited her on the date. We have known her as an acquaintance / casual friend for a few years prior to this development.