how to change every 3rd character

hey champs,

i have a files, whose contents are as follows,

abcdefghijk
lmnopqrstuv
..............
..............

i want to replace every other 3rd character to some specified character.
let here in this file i want to replace each 3rd character to z.

abzdezghzjk
lmzopzrszuv
................
................

so can anybody let me know how to achieve it ????

sed 's/\(..\)\(.\)\(.*\)/\1z\3/g' pattern
I think there exist simpler method

I think the OP wants this:

sed 's/\(..\)\(.\)/\1z/g' data