COALESCE should help you in this case. It returns the first non-null value.
SELECT COALESCE(S.APPROVAL_DECISION,A.APPROVAL_DECISION, R.APPROVAL_STATUS) AS SomeColumn
Or, if for some reason you don't want the first 2 values and your condition is to get the third only when the first 2 are null you can use a CASE statement.
SELECT CASE WHEN S.APPROVAL_DECISION IS NULL AND A.APPROVAL_DECISION IS NULL THEN R.APPROVAL_STATUS ELSE 'WhateverYouWant' END AS SomeColumn