Need a formula to calculate grade gains of students over three testing periods

Hello,

I need a formula that would give the average grade gain for a student. For example, a student will test when they enter our school and it will show what grade level their test scores place them, they are equivalent to the year and month. So, a student may enter our school and test in math skills equal to 7th grade, 8th month, or 7.8. In the middle of the school year, we test the student again to see where they are. The score may be equal to 8th grade, 9th month, or 8.9, for a grade gain of 1.1. This is easy enough with the SUM formula, however, I am having a problem with the 3rd testing period. Each school year a student tests 3 times, so I need to show the total grade gain from test one to test three. In other words, in test one the student started at a 7.8 and ended at a 9.8, I need the formula to eliminate the middle test score when the final (3rd) test score is entered. Does this make sense? The SUM works for test score 1 Column C, the middle (interim) score column D, but when 3rd test score is entered in column E, it will not calculate properly.

Using Excel 2010. Any help is greatly appreciated.

Thank you so much, Moo. That did it!
Have a great day!