(2) is correct as-is, but it doesn't mean what you want it to mean - at least in the US, it would be taken to mean you were in the fourth grade of primary school, so you would be somewhere around nine or ten years old. (1) would be correct if you changed it minutely to "I'm a fourth grade r " but would have the same meaning as I said above. (3) would mean what you wanted it to mean if you changed it to "I'm a fourth- year college student".
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