Seville-
I stayed at Hostal Londres . It's very nice, quite and homely. The facilities are great (private bath, phone, TV). Rooms were clean and well spaced. Receptionist is available 24hrs but hardly speaks english. It's well located to anywhere you really want to go.

Madrid-
Hostal Martin . I liked this place. It looks shady from the outside and by the looks of the lobby but very nice once on the hostal martin floor. The rooms are nice. I had the balcony view. 24 hour receptionist and they speak moderate english. It's a busy place during the day but quite at night. Close to plaza mayor/sol. They have a safe that costs 1 euro to lock everytime you open it. And there is a nice TV.

Barcelona-
I couldn't find a place in barcelona so I ended up staying in cornella (a city just outside). I stayed at the NH hotel Cornella . Very nice hotel but far from the city and since the metro stops running at 2am, you have to make sure u get back in time or take a taxi. Nice hotel staff. clean and very nice bathrooms. Free safe (electronic). Big TV. Located right by the mall for those who love shopping. This place is nice but just too far from the city.

A couple of questions...

Did the hostal in Seville have air conditioning?

Also, for Hostal Martin, how far in advance did you make a reservation? I am going to be in Madrid in mid July and I'm struggling between not wanting to be pinned down to reservations and a strict agenda versus wanting to have a good place to stay without having to walk around the city to find a place to stay. Hostal Martin seems to have good reviews. Anyone have an idea whether I should book now or wait?

All the hostels I stayed in had AC. So it's no problem. The AC in hostel martin works with a remote control kept at the front desk. So if you want AC just call in and the person will come turn in on for ya. I stayed in room 111 which is right next to the front desk so I felt safe. Hostel Londres has AC with the control in the room by the beds.

I am not sure how far in advance they need reservations to be booked but I am sure you can work in and if they have space they will give you. I made the reservations about 2-3 weeks in advance though. They also have hostel cervelo on the third floor (same building) so you can ask them about that too.