perl -054pe"$.%3||s/,/\n/"

##</code><code>##

>echo a,b,c,d,e,f,g,h,i | perl -054pe"$.%3||s/,/\n/"
a,b,c
d,e,f
g,h,i

##</code><code>##

perl -l054pe1 | perl -054pe"$.%3||s/,/\n/"

##</code><code>##

>type file
a,b,c,d,e
f
g
h
i

>perl -l054pe1 file | perl -054pe"$.%3||s/,/\n/"
a,b,c
d,e,f
g,h,i