How many days in each city

Which city(ies) do you fly in and out? It makes a huge difference.

For example, if you fly into Porto and out of Faro, or vice versa, you could do all three, just barely.

On the other hand, if you fly in and out of one city, I would suggest that you visit only 2 cities and forget the third. Too much doubling back, travel and hotel changes to do the third. There are lots of day trips you can do from any of those 3.

Does eleven days mean twelve nights in PT? Early May is not the time to visit the Algarve. The summer months is when you want to go nor would I choose Faro as a base.
Lisbon alone deserves four nights and that does not include day trips to Belém, Sintra or Évora. If you add day trips add an additional night in Lisbon per trip.
The same with Porto. If you day trip to the Douro Valley, you’ll need to add another night. There are other places to visit from Porto too and Porto needs two nights. Can you fly in and out of Lisbon and Porto?

We spent 3 nights in Porto and felt it was sufficient for what we did.