=IF([Risk Score]<=3,"Low",IF([Risk Score]=9,"High","Medium"))should work out what is a high medium and low risk. The post below this, from endusersharepoint.com is fine and lays out in excruiting detail all that is necessary to do anything with calculated columns. However the above solution for nesting columns looks easier on the eye, and mind or at least my eye and mind and sort of explain itself really.
Nested if statements in calculated columns
Assuming that you have a column called severity and likelihood and you've multiplied them together in column called risk score:
