As Cambridge dictionary explains, when it describes something damaged, broken can be both predicative and attributive adjectives.
1. He put a sheet of plastic over the broken window. (attributive)
2. She had an X-ray to see if any of her ribs were broken. (predicative)
Regarding the third sentence, you need to use an article before the adjective.
However, you can consider be+broken as a passive construction, but in the passives the agents are the key elements. In other words, you use passives when the agent is more important than the subject. Needless to say that, be itself, is a linking verb which can be followed by adjectives.
I think if your examples were with more details, then it would be more easy to judge them:
- This window is broken by little pressure. I don't like it! (broken is a verb)
- This window is broken though it couldn't be discerned easily. (broken is an adjective)