Which of these is the correct usage- 'I will call you once in India' or 'I will call you when I am in India' or 'I will call you when in India'?

All of these sentences are syntactically acceptable English.

However, the first and third are not colloquial. They sound somewhat strange to a native user of English, the first one especially so.

A good sentence is the middle one:

I will call you when I am in India.

or something along the same lines, e.g.

I will call you when I reach India.