Really, you do what the three of you want to do. Whatever you are comfortable with. If you want people to know, you act casual and answer questions honestly that come up. If you don't want people to know, you let them wonder.
I personally am going to have no problem telling people that I'm poly, now that I'm actually exploring it. I enjoy being open with people and very much live by the philosophy that people don't have to like me and if they don't, sucks to be them because I'm awesome.
I also like being open about these sorts of things because by being open, I know that someday I will change someone's life, hopefully for the better. Maybe give someone the courage to find their own path to relationship happiness. I don't think things like this should have to be hush-hush.
That being said, there are a few people I will never ever tell. My family, for instance. They are very devout Jehovah's Witnesses, and the consequences of me being unnecessarily open with them aren't worth it for me at this time.
My secondary told his mother, who was ok with it, and then completely supported it after she saw how happy her son was being with me.
My primary doesn't feel comfortable telling his family, for reasons I'm not quite sure of yet, but I don't blame him.
Just remember to always respect each other's comfort levels in this area. I would be very upset if one of my lovers just disregarded my wishes to not tell my family. It would mean being completely disowned. So, just as a cautionary, if one of your partners isn't too keen on being as open as you are, please respect that!