Hello! My husband and I will be going on our honeymoon in mid-September. We will be going to Rome and the Amalfi Coast for the first part of the trip, but wanted to add one more country to the last part of our trip. We are deciding between Lisbon, Barcelona, or Croatia. We will only have 3-4 days to explore one of these cities. Any help would be greatly appreciated!

I realize this is not answering your question, but some questions that will help.
Where else have you traveled?
Which places have you enjoyed the most?
Which is more important to you? Beautiful landscapes, vistas, coastline or amazing architecture and buildings? Art, food, etc?
Have you been to other places in Italy?
How much time are you allowing in Rome and on the Amalfi Coast?
Did you consider any other places in Italy?
In planning are you allowing for one day and possibly one night of travel at the beginning, more in between places and at the end?
Note: with three days, you are not really going to another country, as in Spain or Portugal. You would be going only to one city for a brief visit. You don’t want opinions based on the thought that you might be going to see the country.
Have you already booked any flights that you must take into account?

To point out the obvious, Croatia is not a city.

Oh, yes, of course, you are right.
I meant that the OP titled the thread Spain, Portugal, and Croatia, but in her post said only Barcelona and Lisbon, along with Croatia. Just noting that the title might be interpreted to include more for consideration, though I think she did create a poll for choosing. All great choices for different reasons. IMHO, a poll is not useful for picking a place unless it offers reasons for the choice.

Welcome to Fodors. I'm not sure a 'Poll' is really helpful for this sort of question/topic. All of Sassafrass' questions are really important before we can offer useful advice.

Thanks

Obviously. Thanks for your insight.

We enjoy landscapes, architecture, and food. We will be spending 6 days in Rome, 3 days in Positano, and 4 days in Ravello. We know that we could explore more of Italy, but we'd rather see another country. We only have 1 flight booked to Rome and then we plan on flying out of Naples. We know 3-4 days is not enough time to explore a full country we just wanted to explore one more city.

Given your interests, particularly in architecture, I would probably lean towards Barcelona. But all three places have much to offer and all three should be really nice in September. So I might decide based on whether it changes your flight back in terms of making it easier or cheaper.

Barcelona tends to fill up with conferences in September, but this year.... who knows

I know them all and I'd go to Croatia

I would do Barcelona. Vueling Airlines flies between Barcelona & Naples: https://www.vueling.com/