Excel Formulas

I'm having trouble finding the excel formula for:

Add a column that shows the total of each person's best three scores out of their total four scores.

And I'm also having trouble with this one:

Add a column that gives each person a grading using the information in the table.

Total Score
(4 rounds) Grade
Under 306 A
306 - 310 B
Over 310 C

Example:
78 78 83 80 = 319

For this one I tried: "=IF(SUM<306, "A",IF(306<SUM<310, "B",IF(SUM>310, "C")))" It didn't work but I don't know why >.<

Any help would be extremely appreciated, I've already tried googling it but I've had no luck.