I know we can answer either :
- No, it's not
- No, it isn't
But is it accepted and understandable to write :
What about saying it ?
An acceptable, if somewhat archaic, contraction would be
You are asking if it is acceptable to write:
The answer is: no, it is not acceptable.
A contraction is normally (and traditionally) of two words, not three. So when you intend to say "it is not" you can either contract "it is" to "it's", or "is not" to "isn't".
Exceptions to this would fall under the description of nonstandard, colloquial or dialectal contractions - where regional dialects slur words together so they sound like a contraction, but strictly speaking they should not be written as such. True, some writers of literature use artistic licence and make their own written representations of such dialects. Generally, though, these would be pronounceable phonetically. I suggest that your example of "it'sn't" is neither acceptable according to the rules of grammar, nor is it a representation of any dialect. It looks like it would be pronounced as "itsent", and that is not anything I have ever heard as a well-travelled native British English speaker.
It is neither accepted nor understandable to say or write that. Say it isn't (2 words). Or you could say "it's not".
"It is not" can be contracted into a single contraction. This contraction is different from the one proposed by the original poster. According to Merriam-Webster, "'tain't" is at least 245 years old. Because it includes "ain't", it is not Standard English.