Treat "," as the end of line marker (-054).
Every third line ($. % 3 == 0),
change the trailing comma to a newline (s/,/\n/).
Print the result (-p). 

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

[download]

In action: 

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

[download]

Update: As moritz points out, I assumed all the input is on one line.

In reply to Re: How can I make it to a one liner by ikegami
in thread How can I make it to a one liner by greatshots

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