How can we auto. set "Schedule End Date" on the Task which is also -3 Days from the Request Dueby Time?

Use Case : We are setting (assigning) SLA on the Service Request dynamically based on the request additional field value (picklist) and based on the "Dueby Time" set on the Main parent service Request how can we also assign Schedule End Date on the Task which is also less then 3 Days as per main service request dueby time?

Regards,

Wajahath Farooq