My father died recently after being bed ridden for 5 years. His namaz for 3 years are pending. Please advise how can I offer namz on his behalf.

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As usual, 5 prayers for a day, and you can pray as much days you can every day. This means you can pray the prayers of 5 or 10 days or more or less, per day, till you complete the 5 years.
However, if during the 5 years he was not conscious, you do not need to makeup the prayer of the period in which he was so.