Why do we chant ‘Om’ three times?
Om is made of 3 syllables ‘Aa’, ‘Uu’, ‘M’. That is why we chant 3 times.
You can say that it is for the lower body, middle body and upper body, like we do three stage pranayama.
If we do it 4 times, even then you will ask me why 4 times? If you want you can do it 4 times. Whenever you choose a number there need not be any reason for it.
You want to do it 5 times, you can do it.