Could you recommend a comfortable yet cheap 3-star hotel in Amsterdam?

There are several in the southern part of Amsterdam. It's easy to find the 3* hotels in that area with a rating via a portal that specializes in Holland. Amsterdam is really a small city with excellent tram and subway, so don't focus on the center but get one in the southern part. Choose one that is close to a tram station, say nr 5, that will get you in the center within 10 minutes.

I stayed in a small hotel in Haarlem which is a 20 minute train ride from the airport. Very nice, very quaint and much less expensive than the big hotels. Unless you're looking for nightlife. If that's the case, a hotel in Amsterdam will best suit you.