Is my father's missing three prayers before his death due to severe illness and intensive care considered a bad thing, and what can we do to expiate these prayers?

If your father lost his mind during that period, then prayer is not obligatory for him. If his illness became so severe that he was unable to gesture (for prayer), the majority of scholars are of the view that he was still obligated to pray according to his ability, whereas the Hanafis believe that prayer is waived for him in such a condition. Since your father has passed away, we hope that he is excused. No expiation is incumbent upon you, but rather you should seek forgiveness for him and supplicate for him. If you give charity on his behalf or perform any act of worship and dedicate its reward to him, that will benefit him according to the majority of scholars.