3 nights isn’t very much. You can easily spend that in Vienna or Budapest. Bratislava js small and can be seen in a day. The castle isn’t the prettiest either.
For the places mentioned in Czech, they’re too far and not worth it, unless you wanted to spend 3 days in Prague before flying home.
But if you’ve never explored Vienna or Budapest, I’d stick with those.
If you want something outside of major cities, then you can look around Valtice/Mikulov, which is a wine growing region. Lots of bike trails, etc, and not far from Vienna. There’s also the Lake Balaton area in Hungary, but I’m less familiar with that area.