For goodness sake Tara!
If they said 2-3 weeks, and you are concerned that they haven't notified you one day after the start of that 7 day window, you need to find a distraction.
The fact that you had an interview means that you will receive a reply from them. If after another week or so you have still heard nothing, then by all means telephone (whoever interviewed you's) secretary and politely ask if anything has been sent out to you? You can always say you have been having difficulty with your post, and wanted to check that you hadn't missed anything. However if you start chasing them up now, when they are only at the earliest point of their planned decision and response period, it might appear a little annoying.
Although it not always the case, companies do often telephone successful candidates with the good news, prior to sending a letter of confirmation, however that is not a prerequisite and a letter would be sent in any event.