Luke wrote...
Yes, a bit complicated. Formula in B3 (fill right) returns #VALUE! If I
jockey the formula around a bit and put it in C3 (fill right) I get all
blanks in row 3.
....

My fault. Didn't test. The col B formula can't be the same as the col C
and further right formulas.

B3:
=IF(ABS(SUMPRODUCT(--(YEAR(B$1:$IV$1)=YEAR(B$1)),B$2:$IV$2)-30)<=3,
SUMPRODUCT(--(YEAR(B$1:$IV$1)=YEAR(B$1)),B$2:$IV$2),"")

C3:
=IF(AND(SUMPRODUCT(--(YEAR($B$1:B$1)=YEAR(C$1)))=0,
ABS(SUMPRODUCT(--(YEAR(C$1:$IV$1)=YEAR(C$1)),C$2:$IV$2)-30)<=3),
SUMPRODUCT(--(YEAR(C$1:$IV$1)=YEAR(C$1)),C$2:$IV$2),"")