The 75% and the 3:1 ratio are not related. Methane has the formula CH4, so for 12 grams of carbon you have 4 grams of hydrogen. If you successfully break down the molecule CH4 you get a carbon-hydrogen ratio of 3:1. Now, let's say you start with 5.33 kg of methane. Only 75% gets converted, so that's 4 kg. Of that, you get 3 kg of carbon and 1 kg of hydrogen.