How much of the 8% solution should we use to make 100g of a 3% solution

Assuming the stock solution is 8% by weight, then

100g of 3% solution contains 100*3%=3g of pure substance.

Amount of 8% substance needed is 3 / (8%) g = 3g / (0.08g) = 37.5 g of 8%

mixed with (100-37.5) = 62.5 g water.