After 3 weeks maximum capacity got down 3%

"Depending upon the length of time between when the iPhone was made and when it is activated, your battery capacity may show as slightly less than 100%" iPhone Battery and Performance - Apple Support

Yes.

However, I would contact Apple Support and have them run a wireless hardware test on the battery to see what it says. At the very least, you have a documented issue and can later on argue in store that the battery needed to be replaced too soon due to Apple's marksmanship. Or something along those lines.

Apple won't do anything unless the battery health dips to 80% in the first 12 months.

Battery health is not precise. It changes based on when it is checked and what the phone was being used for at the time it was checked. Unless there is a clear degradation in performance, battery health should be checked a few weeks before the one year manufacturer warranty is up. Constantly checking it is pointless, to be honest, especially since it can change.