Auto complete a field from data filled in to two other fields

I have a large table in which I am entering assessment information

The last three columns of the table are drop down menus. This is for
about 60 rows.

1. is U,Q,L,H
2. is N,M,S,E
3. is L,M,S,C

I would like a macro to auto fill column 3 (instead of having the work
out the result for the matrix table below) from what has been filled
in for columns 1 and 2

e.g. 1= Q 2 = S therefore 3 should = M


N M S E
U L L L L
Q L M M M
L L M S S
H L M S C
Can anyone help ?

This is the matrix for the results I need in column 3.