@Alexander has already provided a good answer which I have upvoted. He has explained the meanings of the three words which you were considering. I would like to expand on that base by talking about what would happen if you used each of them in your sentence.
My experience watching Americans learn Russian is that they start with translating by word substitution. So they say "For is для." Then they find out that that is not always true and so they start memorizing 'exceptions' and building decision trees. But this quickly becomes unmanageable. Advanced learners get over this hurdle by gaining a real understanding of the picture drawn by each Russian word.
A good exercise to build this skill is to plug candidate words into a sentence and try to figure out what the sentence would mean. If the result is nonsensical or not what we wanted to say, we discard it. Let's try that with your sentence and your candidate words.
Свобода не за нас.
While this literally is "Freedom is not for us." it is in the sense "Freedom is not acting in our behalf." It is "for" in the same sense used by the Apostle Paul at Romans 8:31: "Если Бог за нас, кто против нас?" (If God is for us, who is against us?)
Свобода не про нас.
This would seem to mean "Freedom is not about us." Like @Alexander I can't think of an expression in English where "for" could be translated "про".
Свобода не для нас.
This means "Freedom is not for us." in the sense that freedom is not something we get to use.
We use "для" to talk about how something will be used or who gets to use it:
Этот подарок для вас. (This gift is for you.)
Эта книга для вас. (This book is for you.)
Это приспособление для открытия банок. (This is a device for opening cans.)
You may also be interested in an article I have written about translating "for":