I will have been travelling for 3 weeks when I visit Granada and by then I will need to wash my clothes.
Does anyone know a hotel with a laundry or a laundromat, chinese laundry or other place where I can wash my clothes?

There are a couple of laundry places (laundromats) in Granada for example

Tintorerías Niágara.
They charge 5.80 euros for a 6kg load.

General info:
https://tintoreriasniagara.es/servicios/

They have several branches in Granada
https://tintoreriasniagara.es/tiendas/

 

Normal Price for a Self-Service Laundromat (Lavandería Autoservicio) in Spain

If your goal is to wash a standard 6kg load of daily clothing yourself, you will want to look for an autoservicio. Prices across Spain are highly consistent and generally fall into the following ranges:

• Small Washers (6kg – 8kg): 4.00 € to 6.00 € per cycle. This usually takes around 30 minutes and frequently includes professional-grade detergent and fabric softener automatically dispensed by the machine.

• Medium/Large Washers (10kg – 14kg): 6.00 € to 9.00 €, ideal for large family loads or bedding.

• Dryers: 2.00 € to 4.00 € total. Dryers are typically billed in timed blocks (e.g., 1.50 € to 2.50 € per 10–15 minutes). A standard load usually requires 20 to 30 minutes to dry completely.

In total, a complete wash-and-dry cycle for a 6kg load at a standard Spanish self-service laundromat will cost between 6.00 € and 10.00 € and take about an hour. Many modern franchises also offer a downloadable app or local loyalty card that knocks 10% to 20% off these standard rates.

The laundry service in Hotels is very expensive.
Unless you have money to waste it may be a good idea to use alternative methods.

Many short term rental apartments have washing machines included in the price so
washing your clothes is free.

Most places don't have clothes driers because it is so easy to dry the clothes in the sun.

More info about Landromats in Spain

https://infoaboutspain.com/article/lavanderia