This is about calculating the average of the two highest scores.  If using VBA code is okay here, how about this approach?  Calculate all three averages, then use IF THEN statements to test for the highest average and use that?  Like this:
Average1 = (F1+F2)/2
Average2 = (F1+F3)/2
Average3 = (F2+F3)/2
If Average1>Average2 then
   TempHighAverage=Average1
Else
   TempHighAverage=Average2
End If
If TempHighAverage>Average3 Then
   HighAverage=TempHighAverage
Else
   HighAverage=Average3
End If