Question:
Three weeks from now will still be low-season. You can either go to theAmsterdam Tourist Information office on Platform 1 of the Central Station,or just search for a good hotel on the net (or ask here forrecommendations), write down the telephone nr. of the hotel and call themwhen you arrive. Or just make a reservation!I don't recommend the "hotel pitchmen" that are sometimes to be found at CS.

Answer: Sjoerd may be close to the action than I, but I would recommendmaking a reservation. Even if it is still "low season", Fridayafternoon is when all the weekend trippers arrive, and therecan be long lines and/or poor selection at the office at thestation.

In my experience, it is not uncommon to see many full hotels incentral Amsterdam on Friday and Saturday, even deep in the offseason.