When to use which word? obligatory, mandatory, compulsory
From my understanding all of the words listed in the question are of the same meaning.
However, I do not know if there are situations where I have to use a specific one of those terms.
Are they the same? Are all of them being used commonly?

Question

When to use which word? obligatory, mandatory, compulsory
From my understanding all of the words listed in the question are of the same meaning. 
However, I do not know if there are situations where I have to use a specific one of those terms. 
Are they the same? Are all of them being used commonly?

Answer

Same but with slight differences...

Mandatory - required by rule or law.
Obligatory - required by customs and tradition or out of loyalty etc.
Compulsory - having the power of enforcement, able to force someone...

Answer

They are synonyms and can be used interchangeably. :) They are all commonly used.