My dear friend, it is illegal. You cannot hold anybody with this type of systems. If you want to obtain a bond, it should be two-way, i.e., if you want a person to continue with you for three years, the same is applicable from the employee's side also. You cannot ask him to stay for a minimum of three years, which is not feasible. The exception is in the case of substantial investment in an employee for his development/training, which should be clearly visible and provable in a court of law if required to bind; otherwise, you cannot. Hence, please concentrate on making employees take ownership of their responsibilities and jobs within your organization, rather than resorting to bonded labor systems.
Please work out an action plan on how to retain an employee, starting from job description, role and responsibility, growth/career paths, training, development, creating a happy workplace, family orientation, incentives, rewards and recognition, delegation of authority, and work-life balance. For all these aspects, you should have patience, an information inflow mechanism, openness, etc.
Thanks and regards - Kameswarao