first: 1:3 is equal to 1 part in 4 parts total, or 1/4 =25% v/v

1:12.5 is equal to 1 part in 13.5 parts total, or 1/13.5= approx 7.4% v/v

multiply 0.074 * 0.25 = 0.0185 or 1.85% v/v or approx 1.9% v/v

then to see what the overall dilution factor is:

solve the equation for x

1/(x+1) = 0.019 x = 51.6

so overall this is equivalent to approximately 1:51.6 dilution

hope this helps,

JBJ