Has he/she asked you to do so verbally or in writing?
1. The notice period of 90 days may not neccessarily be applicable to you
................and may not be more than 30 days......................hence the company norm may be illegal.
2. The Manager or even employer can ONLY REQUEST you and can not ASK/ORDER/IMPOSE to extend the notice period beyond 30 days ( if this is the notice period as per various enactments ) or 90 days as inserted in appointment letter.
The request made by Manger/Employer is subject to your approval.
Be Careful to get such request in writing under proper seal and signature of competent employee i.e. appointing authority........
Here you have full discretion to negotiate your wages/CTC etc for the extended period.
3. Before you separate communicate in writing under proper acknowledgment that NO Tasks are pending at your end and to whom you should handover the charge within your last date in office i.e. dated......................under proper acknowledgment on the spot.