ArturoK, I'd recommend contacting all three of these ASAP to simply check availability. They're often booked on weekends for at least a month in advance but not sure when you'll be there.
I've stayed at all three of your aforementioned hostels (called "hostales" in Spanish/Spain). Hostal López and Hostal Dulcinea are located about one or two streets apart in the HUERTAS neighborhood (my favorite) and one is extremely similar to the other, both simple, clean, basic lodgings and kind owners. Hostal Cervelo is on the busier Calle de Atocha not far from the Plaza Benavente.
Regarding
Hostal López, conduct a SEARCH to find dozens of reviews about them or consult
their website or send them an email with specific questions.
Hostal Duclinea, I don't recall that they had an elevator so you'll likely have to walk upstairs to the reception room. Decent, clean rooms with simple furnishings too, television, small bathrooms like the rest, a very typical, old-style hostel. There too you may well have a balcony if you have street-facing room. They accept credit cards.
Hostal Cervelo is on Calle de Atocha and is more modern, the building has an elevator, the rooms have TV, wireless internet connections (with your laptop), telephone, and some decoration, air conditioning (which you won't need in October), a lock-box, and a small bathroom. They also accept credit cards.
Again, be sure to check availability for these three NOW. Good luck! Enjoy your visit to Madrid.
Saludos, MadridMan