Join lines



Hi,

I have a file with a lot of lines with a singer number like the
following:

111
222
333
444
555
6666
77777
888
9999
..........

How could I join 3 lines at a time and with a "," in between and at
the beginning of the next line, as in the following:

111,222,333
,444,555,6666
,77777,888,9999
.........

Thanks in advance.

Nick Li

.