Is anything required of me for not having prayed over my deceased grandmother three years ago, given that I was unable to pray over her at the time of her death?

Being dutiful to one's grandparents and showing them kindness is akin to being dutiful to one's parents. One can compensate for their shortcomings in being dutiful to their grandmother after her death by praying over her grave and supplicating for her. Imam Al-Shafi'i and others permitted praying over the deceased in their grave, as evidenced by the Prophet's (peace be upon him) prayer over a deceased person in his grave, and the prayer of some Companions and Tabi'in over graves after burial. The correct view is that it is permissible to pray over a grave even if a long time has passed, as long as the person praying was eligible to pray at the time of the deceased's passing. Furthermore, the greatest way one can be dutiful to their parents and grandparents after their death is by supplicating for them and seeking forgiveness for them.