Plop this into C12 and copy over.

=SUM(OFFSET('Pull 1'!$A$7:$A$25,0,MATCH(Summary!$B$9,'Pull 1'!$B$3:$BG$3,0)+COLUMN(A1)-1))

This takes the range of A7:A25, and offsets it by the number of columns where it finds a match for the validation, and then sums. The column(a1) part just increments the offset by 0, 1, & 2 to pull the other parts of the group.