I would like to more accurately calculate an overall student grade. At our school we grade students from A+ to E-. I've tried assigning these grades to a numerical value (A+ = 15 down to E- = 1). Using these numerical values I can average results and come up with an overall grade. In most cases it works quite well but my problem lies in the ruling we need to adhere to when there are three assessments.

Overall A Standard = A in any two assessments and no less than a B in the remaining assessment
Overall B Standard = B in any two assessments and no less than a C in the remaining assessment
Overall C Standard = C in any two assessments and no less than a D in the remaining assessment
D = At least Standard D in any two assessments and an E in the remaining assessment
E = Standard E in the three assessments

Note: A, B, C, D and E also includes A+, A, A-, B+, B, B-, C+, C, C-, etc.

So for example:
B+, B+, A+ should = B+
A-, A-, B- should = A-
A+, A+, C+ should = B+
A+, A+, D+ should = C+
D+, D+, A+ should = D+
E+, E+, A+ should = E+

Hopefully this makes sense. Any ideas or suggestions would be much appreciated, thank you.