Hi
These types of hol are usually fine if you're just looking for cheap get-away. It may not actually be 3 star, Airtours use their own 'A' rating so it will be a minimum 3 A's. You may get lucky and find yourself in a 4 or 5 A property! Whatever happens they can't allocate you anything lower than a 3 A and it must be in the resort specified. If it just says 'Algarve' you could be in any of the resorts but pick up an Airtours brochure and you'll get an idea of the sort of place you could end up in although it doesn't have to be one of the properties in their collection.
If you're not happy when you get there make sure you tell the rep but be aware that any complaints won't have much pull if they're to do with things like building work, or steep hills and steps etc as this is the main risk you take when booking un-named accom. Obviously the accom should be clean, safe and secure and these things should be dealt with by a rep immediately regardless of how much or how little you paid.
I'm sure you'll have a great time though, i know plenty of people that book this way and usually end up in a higher rated property. Have fun!