As Vaishalee Parkhi and Pon have mentioned, in addition to providing complete organizational information to this forum for advice, please also inquire about the facts of the case.
Another aspect is, assuming you have a formal HR policy in place for Annual Appraisals, even the Manager is bound to follow them. If the employee is not performing well, then I am sure the employee needs to be counseled and, in the worst-case scenario, let go — as per the laid-out policy, not based on the whims and fancies of any official. If the employee is disliked by the Manager for personal reasons, HR has every right to intervene — with the focus being on the larger company's interests and not the manager's.
From a financial perspective, it's the company that is paying the employee's salary — not the Manager. Therefore, it's neither the Manager's prerogative nor right to waste company resources on an unfit employee. Frankly, if I were you (and you have every right as HR to ask such questions), I would first question the Manager for delaying a decision on the issue for 3 years. Based on this fact alone — that you mentioned — I think there may be more to the situation than meets the eye with regard to the manager. Please conduct your checks (if necessary, discreetly) and figure it out.
If you address the issues raised by Vaishalee Parkhi and Pon, members could provide more accurate guidance and suggestions.
Regards,
TS