In general, there's no penalty for showing up with fewer people than booked, as long as at least one person shows up. The only reports I've seen of that being a problem was with special dining packages, specifically the Rose & Crown Harmonious Dining Package. There were reports of people booking a table for 3, showing up with 2, and having to pay for 3.
going from 3 to 2 can be a lot easier than going from 2 to 3 as that can involve a table size change. for most reservations going smaller no problem but a few as others have said to watch