There is loads to see and do in Amsterdam so you won't have trouble filling three days.
Check out the various discount tickets/transport tickets available, though if not using public transport much it may be cheaper for you to just tap on and off.
Not sure if it's already been mentioned, but the Vondelpark is lovely to walk around and is pretty near the main museum area.
In a different part of the city, on Prinsengracht, is the Amsterdam Tulip Museum. Very easy on the eye and with a nice gift shop.
If a big department store appeals, there's the Bijenkorf on Dam Square; handy for the free toilets on the 5th floor :-) The Royal Palace is on Dam Square too.
As PP have said, the red light district is very easy to avoid as it only covers a couple of streets.
From behind the Central Station, you can get free ferries over to various parts of Amsterdam North/Noord, an up-and-coming area.
Agree that, if you do go further afield, Haarlem would be a great choice. Beautiful city and easy to get to.
Have a wonderful trip.