Hello,
Yes, it is possible to increase the notice period from one month to three months. The HR department just has to design a Standard Operating Procedure (SOP) for the same, mentioning that as per clause no. XYZ, we would like to inform you all that the notice period has now been changed from one month to three months. Please also cite the reason for this change.
Additionally, issue a letter to all the employees and obtain their signatures on one copy, which should then be kept in their personal files. For new joiners, you can issue new appointment letters stating a three-month notice period.
Regards,