Notice period lodger help

Hello i hope someone can help me, my contract ends on 31 august
I gave notice last week so more than a month,
On the contract there is specified 3 week notice
But now the agency said i can give notice After or on 31 august so my moving out date will be on 21 september
Im a lodger
In my opinion that makes no sense
I starterd the contract last year on 1 september 2020
Was 6 mont contract then i renew for 6 month more

Your bound by the fixed term that you signed up for which if the two terms were continuous would expire end August.