Formula or macro to more accurately calculate overall student grade

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.

Hi Pauley,
Thank you for the suggestion with the formula. It's definitely of some help. I tried it on a 15 point scale to accommodate the +'s and -'s e.g. For an A+ then 15, for an A then 14, for an A- then 13, and so on. I'm not getting accurate results according to the rules we're required to use but I'll try and play around with it a little and see if it will go a bit closer.

Here's a few scenarios:
A+ (15), A+ (15), B- (10) would produce an A- (13) whereas A+(15), A+ (15), C+ (9) would produce a B+ (12)
A+ (15), A+ (15), D+ (6) would produce a C+ (9) and A+ (15), A+ (15), E+ (3) would produce a D+ (6)

B+ (12), B+ (12), A+ (15) would produce a B+ (12) whereas B+ (12), A- (13), A- (13) would produce an A- (13)
C+ (9), C+ (9), A+ (15) would produce a C+ (9) whereas C+ (9), B- (10), A+ (15) would produce a B (11)

I hope this makes some sense. Thank you again.

Hi Bernie

I think your suggestion for the table is a feasible idea but it would be a heck of a lot of cells. I'll try looking more into applying some rules to calculation.

Thank you so much for your help.

Hello again Bernie. It would be much easier to do as you suggest but our education system has dictated the rules to us. I've just provided some extreme scenarios to highlight potential combinations and the resulting grade. However, you're perfectly correct, a straight average would be most fair. For the most part we don't have too many issues but it would be nice to have this automated.