Hey & welcome to GT!

Originally Posted by: bilinmeyen.13

... my new cordoba c3m, the guitar is great for beginners, sound and keyboard etc. everything cool. only problem is strings get out of tune after a short while, mostly the lowest 3 strings ...

[p]That's a nylon string guitar, right? Nylon strings can be problematic! :) You might need to stretch them & be patient as they gradually settle in. Please check this video lesson for tips on dealing that situation.

https://www.guitartricks.com/lesson.php?input=24279&s_id=1990

Best of success with it! Let us know how it goes or if you have any other questions.