How fast can a three masted war ship go?

It depends :-)

In good conditions you could reasonably expect 10 knots, but no more than 15 knots. For sail boats top speed is related to the length of the boat. The longer the boat the faster it can go (ignoring factors such as total weight and sail area) before it starts to plane (front half lifted out of water which does not work in large boats unlike a small dingy or powerboat which can plane with no problems).