Areas to stay in Barcelona/Madrid/Seville

I use AirBnB for my apartment rentals. I like the consistent practice of having AirBnB hold any security deposits rather then handing over hundreds of euro in cash and getting it back in euros I won't be using again. Even of it means paying the rental in advance, I also prefer AirBnB holding my payment until I have arrived and seen the place. In researching and searching through the other sites, I note the host calls the payment shots and they are not all the same.....just saying why AirBnB works for me...
That being said, I stayed in the area closer to the museums and Atocha train station in Madrid...but that is the area I spent most time in. Sol and Plaza Major are the centre of the city, so with it congestion and noise; but if you want ready restaurant, bar and attraction access, they are common tourist areas.
In Sevilla, I stayed on calle Castelar, between Plaza Nueva and the bull ring. Streets are a bit of a maze but groceries, restaurants and buses nearby. Walk to many places, but in local apartment blocks. Other touristsmin the area.
I tend to prefer quieter areas for sleeping. What are you wanting from your neighbourhood?

Barcelona-Eixample or Barrio Gotico
Madrid- Salamanca, Chueca
Sevilla- Santa Cruz, Triana, El Arenal

Barcelona has been trying to crack down on apartment rentals but is nowhere near as far along as Paris. I have no idea whether something could happen between now and May that could throw a last-minute monkeywrench into your plans.

In Madrid, near Plaza del Carmen which is located between Gran Via & Puerta del Sol and, consequently, quieter, but with easy access to both.

For Seville, in Barrio Santa Cruz.

In Barcelona, don't know what to suggest. We stayed along La Rambla in the Gothic Quarter.

Have a great trip.

Barcelona has required hosts to register their short let apartments for almost a year now. 'Legal' apartments list a HUT number on rental sites. For this reason you may be required to pay the city tourist tax in cash to the host. It is a few euro a day per person. They may also request a copy of your passport in order to register you....just like a hotel will. Seville has a registration system, Madrid doesn't seem to have one yet.
Barcelona continues to try and close the rentals not registered and not paying taxes, but I can't see closing those already registered and abiding by the regulations. There is more 'risk', renting ahead in cities with pending legislation. Some hosts cancel bookings rather than register with the cities. If that is a concern, one can book a hotel with full cancellation options as a back up.

Thank you for the comments. it was very helpful in trip planning.
I chose Airbnb in madrid and Barcelona and still thinking about Seville. but bookings.com had some nice options.
The areas suggested was the most helpful.
We will be traveling in may
Thanks

If not too late, the El Born district in Barcelona is fantastic. Typical, real and historical.
As far as Sevilla, returning there for the second time next summer and I decided to stay in Santa cruz this time.

We stayed in Eixample in Barcelona with Habitat apartments

Madrid: plaza Santa Ana, Spain Select

Seville: cathedral area, VRBO

All were perfect locations and great apartments