grammaticality - Short sentence with adjective and adverb

I think that this is a problem of the usage of adjectives and adverbs (that's why I chose this title):

I have a sentence in my presentation, which clarifies that a procedure uses only observations which are known, so I write:

Uses only known observations.

or do I have to write:

Only known observations are used.

or is the long version necessary:

Uses only observations(,) which are known.

(I'm not sure about the comma)