Whether to grant three month long leave or not that you have to decide. Before taking this decision, ask the following questions:
a) Why is he asking for three month's leave? Is the justification credible? Has he submitted any documents or certificates in support of his long leave?
b) What leave does he have balance? Out of three month's, what portion will be Leave Without Pay (LWP)?
c) If he goes on long leave, how will you manage the work? Do you have standby person who can take up his work?
d) Will you employ anyone temporarily to fill the vacancy caused out of long leave?
e) Above all, is he taking you for a ride? By taking long leave, will he work somewhere and take a chance to establish himself at new place? By chance if he fails there, will he consider employment at your company as granted one?
Apart from the suggestion given by Mr Dinesh, please take a decision after checking with his previous history and work ethics apart from the duration in the present employment. Hope he is not indispensable to the company wherein you have to depend no matter what the situation. If he has enough leaves or at least a major portion of it, you may consider approving by asking him to go on leave without pay for the extra duration. If he avails the whole leave he may or may not return though relieving letter can be withheld in such a situation. End of the day take a call by taking pros and cons of the situation and having a replacement ready in place.