Checking the numbers: The mixture is 21% by weight Ca and Mg. So 5 grams is about a gram of Ca and Mg, or equivalent to very roughly two grams CaC03 equivalent. Two grams in 40 liters is 50 ppm or, yeah, a little less than 3 dGH. So the recipe checks out that way.
Are you actually following it? They're saying to mix up 70 grams potassium sulfate, 270 grams calcium sulphate, and 160 grams magnesium sulfate. that makes 500 grams of total mixture. Mix it well. Add five grams of this mixture for each 10 gallons of water to get 3 dGH.
719 mg Mg and 1200 mg Ca comes out to 1919 mg Ca equivalent or about 4000 mg equivalent CaCO3. In five gallons of water, that's about 11 dGH. While that will make some fish very happy, as well as many plants, it's a tad high for soft water forms. It's much higher than the target you've picked.