Advice on entering a polyamorous relationship.

(Sorry for the length of the question, but the situation is pretty specific, and I am pretty conflicted about the whole thing.)

Ok. So. Recently I have begun seeing a girl (let’s call her Tess for the purposes of this question) and she is perfect. I can talk and have fun with Tess unlike anyone I have ever met. I could go on about how amazing she is, but I would probably drown you in the tidal wave of gushy feelings I am currently swimming in.
The other night she introduced me to her best friend, (sue) another amazing girl who seems absolutely awesome and mature and funny etc. she also introduced me to Sue’s boyfriend (don) nice guy, real quiet, pretty cool.
Well, last night I asked Tess to be my girlfriend (like the complete dork I am) she said yes, but that she wanted to make sure I knew something important first. Apparently, Tess and Sue are more than just best friends; they are bisexual and enjoy each others company in all the ways that one might imagine two bisexual girls might. She says that she really wants me to understand because apparently Sue had made a simmilar deal with Don at the beginning of their relationship… which don did not take seriously enough… leading to hurt feelings.
Tess says that she will only be with Sue and I, and this is not (at least to my understanding) a completely open relationship.
I have never done this sort of thing before. I really want to take this seriously so I am trying to figure out how I should uptake this new information.
I also am entirely used to and agree with the feasibility of polyamory. I know many couples, triples etc. that are leading successful polyamorous relationships. Still, I am worried about becoming jealous, or competitive, or selfish. I have never been that type of person, but when I am with someone, I don’t think about anyone else… I can’t really wrap my head around the concept of loving two+ people at the same time … So while I entirely respect her feelings, I don’t really know how to relate.
I want to know how I should think about this, so that I can be with Tess… And I want to know if it is a bad Idea that I even attempt it. I have always understood that if anyone in this type of relationship is even a little unsure, it is doomed fail… I dont really believe that, but still I am scared.
Any advice/ thoughts/ testimonials etc. would be greatly appreciated.