3 days in Lisbon and Porto. What should I make sure not to miss in each city?
Check out the AT A GLANCE section for each city and see what appeals:
https://www.ricksteves.com/europe/portugal
If possible, try to go to Sintra and or Belem when in Lisbon.
I loved Lisbon.
Are you spending 3 days each in Lisbon and Porto or 3 days total for both cities. If you only have three days for both, it might be better to choose just one of the two.