My mother did not pray the last three months of her life due to her severe illness. How can I make up for her missed prayers, if possible?

There is no expiation for the prayers your deceased mother missed while being unconscious in the hospital nor for the prayers she missed earlier in her life. This is because she may have made up what she missed during her lifetime or have started making them up. Therefore, she will not be blameworthy. You should rather pay charity on her behalf as whatever you wish, ask Allah to forgive and have mercy on her and know that Allah is the Ever Most Merciful upon her.