Adrian Kobsin
Hi I've solved situations similar to yours. If it's only 3 cm you can use a grinder with a diamond disk . I have Marked straight lines with 1-2cm space between them. After , start cutting, going deep how much do I need ( in your case 3cm or a bit more). After you finished all the cutouts . Take a hammer and start braking the thin leftover from the cutouts. You will still have small pieces of cement Wich you will need to brake . After to finish grinde the surface to make it smooth and plain. But Remember Safety First. I recommend that you hire an experienced tradesman to do the job. Maybe in some other way. Cheers
