Your AND functions are doing nothing meaningful in the formulae you posted, so you may as well delete them and use just:
=IF(D3="","",IF((AF3>=59.5)*AG3<69.5,"F",IF(AG3<79 .5,"C",IF(AG3<89.5,"B","A"))))
for the first formula, for example. As for the larger problem, it appears amenable to an array formula such as:
=IF((SUM(IF(K3:AF3>=69.5,1,0))=COUNT(K3:AF3))*AG3< 69.5,"F",IF(AG3 <79.5,"C",IF(AG3<89.5,"B","A")))

Array formulae are input with Ctrl-Shift-Enter.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]