Visiting Belgium with Schengen visa for 3 years issued by the German consulate [duplicate]

To expand on the answer by Kutadgubilig:

In principle, a Schengen visa for the "Schengen nations" (or the the national translation of this term) allows you to visit all Schengen states, in any order.

However, you may not misrepresent your travel plans during the application. When your first visit does not match the itinerary from your application, the suspicion that you lied to get your visa might come up and you may be challenged either at the border or during your next visa application.

So one key question would be what happened to the Germany trip. Is it still planned? Can you document that? Did the Belgium trip come up on short notice? Can you document that? If it is a business trip, are you going to offices of the same company? If the answers are negative, you can expect problems.

In theory, Yes.

In practice, No.

Because it is a German visa, most of the countries will not say anything.

I was searching it 2 years ago for my girl friend. Discovered this statistics: Munich (airport) has the highest number of refusal case due the same reason and that was about ~20 people.

The proplem is mostly gettin schengen visa from some countries like Greece and trying to go some countries like Germany. The other way around will not be a problem. Don't worry. Just make your hotel reservations and convince police officer that you are just a tourist.