It's not bad luck to tell anyone before 3 months, but some people choose not to as the risk of miscarriage doesn't fall bellow 5% until after the 12 week mark. Some people choose not to tell anyone, in case they do miscarry... However others choose to tell people, on the principle that they don't want to think negatively about the chances of miscarriage happenening to them, and they reason that if they do miscarry then at least they will have the support of family and friends - rather than having to tell family that they were pregnant and miscarried, or keep it to themselves... (Others think it would be very private if they miscarried and wouldnt want anyone else to know, which is why they don't tell anyone until the risk drops).
This one is entirely a personal choice - so the only answer to your question, is if you want to tell people then tell the world, if not, don't.