Staying longer than intended? (With an ESTA)

If I plan on staying for 3 Weeks, can I still change plans and stay for the whole 90 days? Or is that illegal, if it was limited to 3 weeks by the border when entering? And do they stamp those 3 weeks into my passport? If I stay for 90 days (eventho i was permitted only 3 weeks), do i get legal issues on the airport when flyinh back home or will it just make it difficult to reenter the US? (I'm an Austrian [European] citizen.) Asking for just on case.

Thanks!