Entirely up to you is probably the only answer possible for this. It's very individual, and people veer from the moment the pee is dry on the stick (or sooner!) right up to avoiding people until they give birth. All ends of the spectrum seem fine to me, so it's a combo platter decision of whether, if something went wrong, would you prefer to deal with it in peace, or have lots of support (but have to go through telling everyone and their dog for weeks to come)?
I've done one whilst the pee was wet, one at six weeks, one at five weeks (then miscarried that one at 6 weeks three days), and I'm currently baking a very quiet stealth bun which I plan to unveil to the world when a leg hangs out. Or whenever I can't fib any more, which is probably at 12 weeks seeing a I have no stomach muscles and get bloaty gassy round tum quite early (TMI).
I think it's up to you. While I was trying I was So strong that I wouldn't tell any one until 12 week scan. I'm 10 weeks and I told every one at work today I told my mum straight away and over the last few weeks excitingly told people bit by bit.
If some thing went wrong I would feel I'd need support from family and friends. We're as some people wouldn't. I do wish I'd kept my mouth shut lol but it means so much I just wanted to tell the world. At least you've only found out I pretty much new at 3 weeks lol so I've found it hard keeping my trap shut.