Three days is not a lot of time.
You could spend 3 days in Lisbon itself. Try to do Lagos and Porto as well and you will be wiped!
I would suggest skipping the others as you will not have a lot of time!
Lisbon has it all - Culture, History, Beaches, Good food ....
When you arrive in Lisbon, stay somewhere central like Residencial Alegria (which has the advantage of being next to a police station.)
[L=Residencial Alegria]http://www.alegrianet.com/[/L]
Then check out the Baixa/Chiado areas, Praça de Comercio, the Alfama and the Castle - Castelo São Jorge.
On your first full day, take the train from Entrecampos station to Sintra and checked out the Palacio da Pena, the forests etc.
On the second day, take the train from Cais de Sodre to Belem or the tram from Praça Figueiras to Belem and check out the Monesterio de Jeronimos (Jeronimos Monastery) and the tower of Belem. After a couple of hours catch the train from Belem Station to Carcavelos or Estoril where you will find beaches. Carry-on later to the terminus at Cascais and finish you day there. Return to Lisbon on the train - 40 minutes.
Note this train line runs right along the Tejo estuary and is a great way to see a lot of the Metro Lisbon area.
Day Three - shopping and flight out
Have a great time!
