I am 3 nights in each... basically 2.5 days and really want to eat and enjoy. :)
If you like historic spots try Meson Don Raimundo in Sevilla -- but please go there at the correct time!
A very popular restaurant in Madrid is Sobrino de Botín. It’s known for its cochinillo (roast suckling pig). Reservations are a must.
One thing I found out visiting those places was it was not hard to have delicious food :)
In Seville, the one that jumped out as very tasty was La Brunilda. A small place and filled up fast. We ate there twice - first when they squeezed us in as we wandered by and then with a reservation at the bar. Definitely would return another time.
We also had done a tapas food tour - really just wandering from tapas place to tapas place and trying a bunch of things. Most of our time in Seville was just that - pop in, eat/drink a little, move on.
In Barcelona, we did similar, but usually for pintxos instead of tapas. We did have tasty but not fantastic tapas at Tapas 24. I think we more enjoyed hitting up one or two places for beer/wine and a variety of pintxos. Super low key and quite a few were really tasty.
Madrid had some good food, but I can't think of any in particular that I would call out. We did go to a RS suggested "historic" place for traditional dishes, and it was fine, but not a likely repeat spot. It was La Casa del Abuelo (Cruz).
In Madrid and Barcelona, it was very common for us to just sit and have drinks & people watch at a few places so that's fun too - just pick a plaza/square with seating and order something. We also ate at some of the restaurants in the large markets like the Mercado San Miguel in Madrid and the Boqueria in Barcelona.
Long story short, delicious food everywhere in Spain, but hopefully you'll get some ideas from this thread too.