Thanks for your comment! The Parliament and the Fisherman's Bastion's are architectural feats so they obviously stand out the most but the House of Terror is a very interesting place to learn more about the city's recent history.
You can definitely visit all three places in one day. The House of Terror and the Parliament are on the same side of the city, in Pest, and they're 25 minute apart on foot. You can then cross the Chain Bridge over the Danube, which is 20 minutes away from the Parliament, and walk up to the Fisherman's Bastion or ride the funicular.
A guided tour to the Parliament costs HUF3500 for EU citizens and HUF6700 for non EU citizens and it's best to book it in advance since it can be full pretty quickly. This tour allows you to visit only parts of the building but it's still really worth it.
The House of Terror costs HUF3000 + HUF1500 for an audio-guide and you can purchase your tickets on the spot.
Finally, the Fisherman's Bastion is mostly free but you'll have to pay an extra HUF1000 to access the upper turrets. There's an entrance fee to the inside of the Matthias Church too, which costs HUF1800.
Hope this helps! :)