How to give excuse before week 12?

it's not really a pantang thing, the 3 months rule is really in place to prevent any awkwardness as the risk is still quite high in first trimester. you can just say it's not convenient for you, if they ask more out of concern, it's OK to just say there's nothing to worry about but if all goes well you can share with them later. it's fine if they get it, it's fine if they don't.