Reggia Caserta - worth it if you only have 3 hours?

I will be staying at Caserta for two nights so have looked at a lot of details about the sites.

Some of the comments said that even with a reservation, the line at Pizzeria Masaniell can get backed up. So, I would pick their earliest lunch reservation time.

The palace has timed entry, also, so pick the earliest one that you think will work for your post-lunch time. Even within the palace, there can be a line to go into the Court Theatre although you will probably miss it since you must be there by 12:45pm (only available on weekends).

I would definitely take the shuttle out to the gardens to maximize your time. Otherwise, renting a bike is what I’ll probably do instead of walking that distance.

You can do it. We did the same last year, lunch at Masaniello, then Reggia Caserta. We would have liked to linger in the gardens for much longer, but it was still a worthwhile visit. However, there wasn't a line or much of a crowd when we went. We didn't pre-book anything other than the lunch reservation.

To reply to your follow up question, no you can't go back in on the same ticket once you go out.
You could get two separate tickets, one for the Reggia palace and the other for the Park, which would work out 3 euros more expensive (9+12 instead of 18).