How do I specify intervals in functions?

Greetings!

In the help file under date and time functions, I see that intervals can
be specified as "interval '3 hours' ". In a PgAdmin SQL window, I can
enter "select interval '3 hours' ", and it will return me "03:00:00", as
expected. I can also enter "select '3 hours'::interval", and get the
same result. Yet neither syntax works inside a function.

declare
ThreeHours interval;
begin
ThreeHours = interval '3 hours'; -- throws a syntax error
ThreeHours = '3 hours'::interval; -- also throws a syntax error
end;

So how do I specify an interval in a function?

Specifically, I'm trying to do something like the following:

if NewRevisionTime < PredictedEndTime - '08:00:00'::interval then

Since both of the shown forms give syntax errors, how do I subtract
eight hours from a time???

Thank you very much.

RobR, who posted this on the novice list but got no answers.