After you do the first weighing between group 1 and 2 and they are unequal it is either because the heavier side contians the odd man out and he is heavier or the lighter side contains the odd man out and he is lighter.
So you have 4 possible H's
and 4 possible L's
A possible H cannot be the odd man out and be lighter…

Separate them into 3 groups of 4
Group 1
Group 2
Group 3