I guess that depends on how you count. If starting from 0, it is 3, or if you don’t count the last week as a full week, it’s also 3.

I’m not sure where you’re from; I’ve lived in various continents - nobody starts counting things like “weeks” from 0.

Birthdays are counted from zero. When you “turn” one, you’ve completed that year.

That's because "birthdays" we celebrate are technically birthday anniversaries. Hence starting from 1 year after the actual birth day

5/7 explanation

If you start weeks at 0, you should do the same for days, 3 weeks = 27 days

A week is 7 days. Three weeks is 3x7 = 21 days.

Also, calendars aren’t exactly the same everywhere. If you say “let’s work out sat and sun every other week”, in the US this results in a calendar where you have every other weekend off. On an EU calendar, this results in working out every week. Or something like that, it was pretty entertaining when my friend pointed out how subtly shifting the calendar to start on a different day results in a very different work out plan.

But back to counting. In the EU, if you go to the first floor, you’d call that the second floor in the US. It has one floor in the EU, but two in the US. I have no idea if this is the same thing or just a mistake. But there are some interesting assumptions in this thread!

3 weeks is pretty well known as 21 days.

Just because elevators and other things start at a 0 doesn't mean that makes sense anywhere else, it's fairly common for ground level to be 0. There is no instance where it makes sense to say 0 week = 7 days, 1 weeks = 14 days, etc. It does however make sense to say I live on the ground level (aka floor 0), I live on the 1st floor (aka second floor).