Flying from Porto to Lisbon to connect to US flight

You have more guts than I do. Assume it is two separate tickets/flights. If everything doesn't click perfectly, your are in trouble. Haven't flown out of Lisbon international so I don't know if the requirement 3 hrs prior is critical or not. I assume also you will have to go through exit immigration and security since you are headed back to the states. What happens if your first flight is 30 minutes or more late? Downside risk is high. I would not do it.

. If we take a TAP air from Porto at 9:00 and have an hour flight to Lisbon

Did you already buy this ticket separately from the one you have from Lisbon to USA?
If not simply just go to Lisbon they day before

I'm a scaredy-cat about such situations and wouldn't try it.

Also, if your flight departs Porto at 9 AM and takes an hour, you will arrive in Lisbon at 10 AM, which is only 2 hr. 15 min. before the transatlantic departure, not 3 hr.

My policy is to be within a quick and affordable taxi ride of my departure airport the night before my flight, just in case something goes wrong with my planned transportation to the airport.

Many things can, and sometimes do, go wrong on the day of departure, which can cause you to miss your connection. What you need to do is ask yourself: if something happens and you miss your flight home, what would the consequences be? How much would it cost you if you had to buy a new ticket (you imply you are on two separate tickets, are they on two different airlines?)? What if there's no more flights out that day, and you have to spend the night...hotel costs, dinner, etc.? Would you miss something important at work at home the next day? Other consequences?

Personally, I've already learned this lesson. If I had two separate tickets on two separate airlines, I'd want a minimum 6 hours or more to make my connection. I would much rather sit in an airport killing time for 4 or 5 hours, than cut it close and stress over trying to make a tight connection (been there, done that, missed the flight).

Everyone has their own tolerance for risk, yours may be different - but I think what you're proposing looks way too risky (and potentially way too costly) to me.

Thanks to all who replied. You are all right. Not a very good idea. Obviously I am a novice at this. I appreciate you taking time and keeping me from making a really bad decision.

We would get to Lisbon the night before and get a hotel at or near the airport. No stress.

My wife and I are Flying from the East Coast to Lisbon
Spending a week there
Taking the train to Porto
Spending a week there
Flying back from Porto

If you are determined to fly round trip out of Lisbon, I strongly recommend taking the train back the day before. TAP has been known to run into delays

Dennis