Matching 3 columns in row - if one column is null then exclude it from comparison
Assuming you want all NULLs to count as equal, I think this will work. But I haven't thoroughly tested it, or even thought about it for very long...

 select A,B,C,'Equal' = 
    case when COALESCE(A,B,C,0) = COALESCE(B,C,A,0)
          and COALESCE(B,C,A,0) = COALESCE(C,A,B,0)
     then 'Y' else '' end
 from @t


COALESCE will pick the first non-NULL value from its arguments, so when testing A against B and then B against C, in each case we look 'forward' for a non-NULL value, with a fallback for when they're all NULL