split a string into 3 characters

Hi,

I have a requirement like this.
String=abcdefghijkl

I need to split this string into a substrign each 3 characters and write output into a file.

Output:
abc
def
ghi
jkl

Can any one please help me how can i acheive this

Thanks

echo $String | sed 's/.../&\
/g' > outputfile

simply ypu can use..

slight modification to this

echo $str | sed 's/.\{3\}/&\
/g'