Something like this: (the columns are assumed to named Q1,Q2, etc)
If a divide by 5 really works in your example (I don't think it will):
=CHOOSE(
INT(
IF( Q1="Low", 1, IF( Q1="Medium", 2, IF( Q1="High", 3, 0) ) )
+IF( Q2="Low", 1, IF( Q2="Medium", 2, IF( Q2="High", 3, 0) ) )
+IF( Q3="Low", 1, IF( Q3="Medium", 2, IF( Q3="High", 3, 0) ) )
+IF( Q4="Low", 1, IF( Q4="Medium", 2, IF( Q4="High", 3, 0) ) )
+IF( Q5="Low", 1, IF( Q5="Medium", 2, IF( Q5="High", 3, 0) ) )
/ 5 ),
"Low","Medium", "High"
Otherwise:
=CHOOSE(
IF( Q1="Low", 1, IF( Q1="Medium", 2, IF( Q1="High", 3, 0) ) )
+IF( Q2="Low", 1, IF( Q2="Medium", 2, IF( Q2="High", 3, 0) ) )
+IF( Q3="Low", 1, IF( Q3="Medium", 2, IF( Q3="High", 3, 0) ) )
+IF( Q4="Low", 1, IF( Q4="Medium", 2, IF( Q4="High", 3, 0) ) )
+IF( Q5="Low", 1, IF( Q5="Medium", 2, IF( Q5="High", 3, 0) ) )
+ 1, "","","","","","Low","Low","Low","Medium","Medium","Medium","Medium", "High", "High", "High", "High")
- One was added because CHOOSE does not work with zero as an index.
- The first "" is for zero (all Q's blank)
- The next ""'s are for if any of the five questions are blank.
- Three "low" for 5,6,7
- Four "medium" for 8,9,10,11
- Four "High" for 12,13,14,15