[megaloman]

Код:

@Echo Off
	Set "Fold=Z:\Backup"
	Set "Mask=*.bak"
	Set /A N=3

	Call :MyDel "%Fold%\" "%Mask%" %N%
	FOR /F "usebackq delims=" %%d IN (`2^>nul Dir "%Fold%" /S /B /A:D`) Do Call :MyDel "%%d\" "%Mask%" %N%
Exit /B 0

:MyDel
      FOR /F "usebackq skip=%3 delims=" %%i IN (`2^>nul Dir "%~1\%~2" /B /O:-D /A:-D`) DO Del "%~1%%i"
      FOR /F "usebackq delims=" %%i IN (`2^>nul Dir "%~1\*.*" /B /O:-D /A:-D`) DO If /I Not "%%~xi"=="%~x2" Del "%~1%%i"
Exit /B