No, Yoong Liat, you are not correct. The first and third sentences are fairly formal and tend to be found at the end of business letters, and the second, with its "I'm" contraction of "I am" is less formal and might be suitable for a personal message to a friend.
I do not understand why the previous poster wrote what he did, and since there is no explanation, I cannot comment further.
Quote:I've heard only the first two sentences, so the third is not correct.
Ah, the "I ain't heard of it, so it must be wrong" fallacy. This is a fairly common error, unfortunately. It is often committed by posters on Able2know, many of them (in theory!) native speakers. Beware of it!
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