3 Weeks in Portugal, Is this too long?

Maybe, but I think the question is why do you want to visit Portugal? Do you want beaches? Food? Art history? Or just a few days to explore Lisbon?

Obviously three weeks spent in Portugal means 3 weeks not spent elsewhere. Portugal is also a bit isolated from most of Europe except by air.

Five days for Lisbon plus day trips to Sintra and Cascais. Hire a car and drive to Evora for at least 2 nights. Then head for a base for a week somewhere around Caldas da Rainha from which in a week you can see the coast plus visit Alcobaca, Batalha, Fatima and Tomar.

Drive to Coimbra for 2-3 nights then head to Porto where you should return the car. Cover the Douro from here.

"What portion should I rent a car for?" Actually, Portugal has a good train system, and there need not be any reason to rent a car at all (though it's usually hard to persuade Americans of this). Most of the popular tourist destinations along the Lisbon-Porto axis, and along the Algarve coast, can be reached by train. I admit that you may want a car if your interest lies in small, quaint, traditional towns and villages, for many of them will not be on the train lines (though some can be reached by bus from towns that are). Also, the train lines seem densest in the long western half of Portugal -- the eastern half, approaching the borders with Spain, are not served quite as well, and so a car could be useful for exploring that region, if that should interest you.

We were in Portugal for 3 weeks in October 2021. Here is our itinerary:
Lisbon - 6 days
Almeirim for stone soup
Castelo do Almourol
Tomar - 4 days
Obidos, Nazare, Conimbriga
Coimbra - 4 days
Aviero
Porto - 6 days
Bussaco, Batalha
Lisbon 1 night near airport
This was our second trip to Portugal. We already saw Evora, Monsaraz, Vidigueira, and Beja. We hire a private driver.

I think 3 weeks gives you time to explore each destination without being too rushed, IMHO.

This was our 3 week itinerary:
Lisbon > Sintra (3) > Evora(3) > Tomar (2) > Coimbra (3) >> Porto (6) >> Lisbon (5) -day trips to Belém, and Cascais.
We traveled solely via public transportation.

Here’s our trip report for more detailed information: Portugal! Our trip in Progress… Sintra

You may also want to refer to this great blog for more ideas: Julie Dawn Fox in Portugal

Enjoy your time in beautiful Portugal!

Three weeks is good, in fact awesome. There's plenty in Portugal to keep you interested and enjoying your time. September is a perfect time to go, too.

3 weeks is definitely not too long. Once you dive into the possibilities, you will probably find, as I did, that the problem is keeping within that limitation. I used the RS guide, which is excellent for the practical advice and self-guided tours. I also used the Lonely Planet guidebook which also covers the eastern side of the country (RS doesn't), which I found as interesting and enjoyable as the rest. I did not go to the south (Algarve) or north or Porto/Douro Valley. You'll want a car for the east. I did part of the west by car, part by train. You will not want or need a car in Sintra, Lisbon or Porto. You can easily visit some other parts by train, though a car may be more convenient depending on your itinerary.

I recommend you start by reading a couple guidebooks to have an idea of what most interests you. This trip report inspired me to look at the east.