I lived as a triad for some years on the Big Isle of Hawaii in a small community. We ran a solar store and especially after a bunch of folks saw us on a network talk show (seems most people had a satellite dish, tho we didn't :) we were very much out in that limited population. We had few to no problems. We never pushed our relationship in anyone's face--only responded when asked and even kept that very brief unless the person obviously was very personally interested due to their own life/relationships.
My practice has been to not tell people anything about my personal life unless there's a reason they need to know. Obviously most strangers and acquaintances don't need to know a thing. Friends vary in their need. Close folks must know. Most hetero couples don't tell their intimate life details to anybody but those close--why should polys?
If strangers speculate it can be very very funny. One guy was sure that one of my partners was my brother--but he couldn't figure which one. When we became a family of 5 adults, everyone thought that my son looked just like his "Dad" no matter which male partner I might have been with at the time they made this comment.
My other technique in being out with friends or with work situations has been to have the people get to know me first as just myself. If they already like you as a kind friend or good worker, it's harder for them to shift their opinion when they find out you are also part of a triad. It would make them have to distrust their own judgement of human nature.
Bio-family can surprise you! Some you might think would be angry will be accepting, some you thought would support you may not. One thing that seems to work for most folks is time. After you've been together a while as three (or more), it becomes obvious that this isn't some temporary strangeness and most families find ways to accept the situation--some by not discussing it, others by just glossing over details of the situation, others by just taking on a "live and let live" attitude. Good luck on this one!
Final note. Yes most people will assume that you two who were first a dyad are either 1. the "real" relationship or 2. are screwing the other partner or just using the new person. Not much you can do to change their suspicions except to reassure them verbally and then demonstrate it in how you actually treat each other in their presence.