Summer Rentals High
Just my experience.....
Tourist areas charge mental prices in the high season, the biggest rental platform is booking.com, then follows air b and b, home and away (Owners Direct), etc etc. Booking.com charge the highest commission at 15% plus payment charges, so naturally these prices are the highest, though big hotels do run special deals to cover their fixed costs in the low season (crazy low - worth a look in those lower months).
You are looking for a 3 month stint which hits the key 'money making months', so you need to shop around for the areas which are not so horrific. I would do a compare/ contrast on booking/air b and b to identify deals on a spreadsheet.
My other half is Spanish - Murica is quite reasonable as I understand, Sevilla you will melt as its soooo hot in the summer, Valencia is a City (not a tourist area and not an attraction as such as the other locations on your list). Girona is similar to Valencia...not a tourist attraction, not something to go mad about.
I have a touristic apartment in Benahavis, close to Marbella (Malaga), and prices rocket, its unbelievable. Often the Spanish rent out apartments on long-term contracts but exclude July and August, as they can charge crazy rates for these two months, anything from €1,500 to €2,500 per WEEK.
Your chosen areas area all very different so you need to ask what to you want to achieve and what you are looking for.
For cheaper rents I suggest looking again at your areas and then look at the lower end rental platforms (as you already noted) like air b and b, Owners Direct, where the commission is lower (3% circa), you could find some deals as there is a mass of accommodation especially generally (again depends in areas). Note a 3 month stint which includes the high season will not qualify for the lower rents ( you got that point). You could negotiate (change your own linens/ mid cleans etc) which might reduce your price.
Hope this helps!