Hello Hanuman
I agree that 'other' is superfluous, and so #2 is better than #3.
However, 'no mountain in the world is higher than Mt Everest' would remain true if every mountain in the world were as high as Mt Everest.
If every other mountain is as high as Mt E., Mt E. is no longer 'the highest mountain in the world'.
I would reword it as: