SharePoint Calculated field - Compare 3 Date columns

SharePoint Calculated field - Compare 3 Date columns
The logic
if ([Revised Due Date] = "")
set val = [Chartered Due Date]
else if([Chartered Due Date]="")
set val = [Requested Due Date]
else if([Requested Due Date]="")
set val =" NA")
the formulae
=IF(OR([Revised Due Date]<>"",[Chartered Due Date]<>"", [Requested Due Date]<>""), MAX([Revised Due Date],[Chartered Due Date]
,[Requested Due Date]), "NA")