Signing a tribute. Which way is correct?
Liz, Fred, and Nicole
or
Liz, Fred and Nicole
Both ways are correct.
The comma before the last item in the series (the comma after "Fred") is known as the "serial comma" or "Oxford comma." Whether one uses it is mostly a matter of preference, or house style. You can find reams of information about it on the Web and elsewhere. There's also this discussion: Should I put a comma before the last item in a list?
Academic writing would require the Oxford comma to clarify the intended meaning of the list. For example, without the last comma after Fred, one could interpret from the sentence that Fred and Nicole are a couple.