1) I'm off
2) I'm leaving
3) I will be going now. I am in a hurry.
I also disagree with Kevin Randall. All three sentences are correct.
"I'm off" is quite colloquial, and is only suitable for informal conversations.
"I'm leaving" sounds quite abrupt. If you're having a conversation with someone and they suddenly say "I'm leaving", this could sound quite strange and even rude.
"I'll be going now" - with the word "will" contracted to " 'll" - is actually the most natural of the three sentences.
Respected Madam, According to your Question:-
(1) I am off - Here off is not a sensable/suitable word it indicates Inactiveness of any thing Non Living Being - Such as The Fan is off. It is a off of office today. There will be a off of my office on Monday due to vacation.
(2) I am leaving. This sentence belongs to Present continuous Tense, This sentence indicates only the ongoing action of the subject which is incomplete.
(3) I will be going now - These sentence indicates the action of someone which will take place in near future in progressive form which will remain incomplete and the next one i am going now- belongs to present continuous Tense- which indicates an action of a subject or some one going on but incomplete.
I would like to explain giving an Example- There is a group of friends who are discussing out of them one wants to leave and asking permission his group but it will be neither permission nor request- I would like to go as i am in a hurry. ( according to me will be right answer)
How is right? (What is right? or Which is right?)
3) I will be going now. I am in a hurry. (I am going now. I am in a hurry.)
"will be" the future tense with "now" is incorrect.
Hope this helps.