Assuming your data starts at col A and row 5 and extends to row 7 paste the below formula in column A and drag and autofill the formula across in the same row.
=sum(INDEX(transpose($A$5:$L$7),(column()-1)*3+1))+sum(INDEX(transpose($A$5:$L$7),(column()-1)*3+2))+sum(INDEX(transpose($A$5:$L$7),(column()-1)*3+3))
If your starting column is different from 1(Col A) then change the offset to the column number you are starting the formula at. So for column D you modify the below from
column()-1
to this
column()-4
The ways this works is to transpose the data array and select one column at a time using index, to be summed. The (column()-1)*3+1,2,3
generates index for next three columns to be summed.