If you are flying directly to Bhutan - Paro, then 3nights you cant see much. But
1. Thimphu and Paro Sightseeing - National Museum ,Rinpung Dzong , Visit Simply Bhutan to understand there culture, Zoo 2. Punakha & Wangdue - Punakha Dzong , Dochu La pass , Punakha suspension bridge ,Temple of Fertility 3. Take a Hike to Tiger Nest- Paro street walk