Barcelona, Madrid, Seville - how many nights each?

50 somethings Hubs and I are flying from the US to Barcelona mid September for 12 nights. We want to immerse ourselves in the culture and see beautiful countryside as well. We like to wander / explore site see, eat tapas, drink espresso and wine, mostly on foot. Train Day trips are great too. We also love warm weather. We like lots of people but not overly crowded or being in lines etc. We will go to a museum or two and a church or two but that’s not our main thing. Being newbies to Spain - Should we spend 4 days in each city? Less time in Madrid and more in Seville and Barcelona? Any recommendations are appreciated. I’ve reviewed a lot of posts and it’s hard to know which areas we will want the most time!