sed - match newlines on input

[bill at hao.UUCP]

I'm trying to match a pattern over multiple lines.  For instance, on the input:

one
two
three

with the sed script:

s/one\ntwo\nthree/one, two, three/g

one would expect to get the following output

one, two, three

OK, so I don't understand the manual (what else is new).  How can I get what I
need?  Also, what about the "Multiple Input-line Functions".  Might that be the
way to go?  An example would really help.  Thanks in advance.

							Bill Roberts
							NCAR/HAO
							Boulder,CO