First you must decide in which country you want to practice medicine - in India, in the USA or in the UK. You have to focus on that particular exam without spending any time on the other one. You won't have any option for changing your decision because any of the 3 preparations will be only of little use to the other.
If you like to practice within India, NEET PG exam is the ideal option. It is an equally tough exam when compared to the other two examinations.
Whereas if you plan to practice medicine abroad, you should be preparing for USMLE for the USA or PLAB for the UK.
Career Grids Services UK Limited, located in London