If you have a clear finish or a transparent or semi-transparent wood stain, then you just need to clean it with a wood deck cleaner and don't have to bother with paint stripper.

Pressure washing, if not done correctly, will damage redwood. It's a soft wood and you can easily be left with a raised grain and a rough surface on your deck.
I doubt you need either pressure washing or sanding since the deck is only 3 yrs old. As leena said, a good deck cleaner is all you'll probably need. It's the only thing we ever used on our redwood decks.

Sanding won't damage the wood like pressure washing could since a sander would remove a small amount of the whole surface, where pressure washing would remove the softer sections of wood.

I would not use a belt sander if you do decide to sand the deck, get a flooring sander.