I spent 3 months there last year with work. I loved it. I would move there in a heartbeat if I was offered a permanent move.
It’s a fantastic city. Lots to do and great bars. They don’t have big bars as a rule they have small places.
My favourite was Cafe Belgique which is just off Dam Square. It’s a great little(I mean little) Belgian bar with fantastic beers. Be warned though most are very strong.
Coffeshops(the smoking kind) are nice and cool. Most are non alcoholic shops but some seel alcohol still. Many bars allow you to smoke but don’t sell. Cafe Belgique is one of them 
3 days is a good amount of time for A’dam as it isn’t a large city.
Things to see.
Anne Frank House
Rijksmuseum
Van Gogh Museum
The 3 things above may mean a bit of a queue though so be warned.
The Heineken tour isn’t that bad and the Red Light District is a late night walk. Just ignore the guys that mumble to you about coke and E and you’ll have no problems.
It’s a very safe city to walk around so I wouldn’t be too concerned about safety but obviously since it’s a city don’t walk around with your wallet in your hand etc. 
You will love it, I guarantee it. One of the most beautiful, relaxed and cool places I’ve ever had the pleasure of visiting.
Oh and if you’re a meat eater Cafe De Klos is a must. Nice pub with amazing ribs. Close to Leidseplein which is one of the main tourist areas and somewhere you will be. Also in leidseplein you will find Boom Chicago which is a comedy club. It’s an American groupo who run it and it is also a must visit night time show.