It was not the action intentional = It was not the action which was intentional = It was not the intentional action.
Is 1st sentence right and is it grammatically correct? is the 1st sentence equal to 3rd sentence?
As a native English speaker, the first sentence sounds very strange to me. I’m not certain if it’s grammatically correct either. The second sentence also sounds somewhat forced and unnatural. The third one sounds the most correct to me, but I think in this case I would say “It was not the intended action”, or “It was not the action I intended”. “Intended” acts like a verb and an adjective here, similar to “The running water” and “The water is running”.