1. The plan was doomed/destined to fail.
2. The plan was doomed/destined to failure.
3. The plan was to fail.
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Are they all the same in meaing?
Does #3 mean #1 or #2?
1 and 2 are fine. Someone made a plan to accomplish something, but people could see from the beginning that it wouldn't work.
3 means something entirely different. This sentence means whoever made the plan intended to fail; that is, he/she planned the failure.