Modifying a textfile

[Olli Virta]

Hi!

So I got this big textfile. It's full of data from a database. About
150 or
more rows or lines in a textfile.
There's three first rows that belong to the same subject. And then
next
three rows belong to another subject and so on, to the end of the
file.

What I need to do, is put the three rows that goes together and belong
to
certain subject, on a one line in the output textfile. And the next
three
rows again on a one new line. And that goes with the rest of the data
to
the end of the new file.

Can't figure out a working loop structure to handle this.

 Thanks! OV