Numbers: How to make a sum product of 2 columns numbers of which only those with checkboxes checked in a 3rd column respectively?
You can add an additional column D, where D1 to D3 would contain:

IF(C1,1,0)
IF(C2,1,0)
IF(C3,1,0)


This would print a 1 if checked or a 0 if not checked, and you simply would need to add column D into your sumproduct:

a1*b1*c1 + a2*b2*c2 + a3*b3*c3