If your citric acid is a powder, then 113 gr (3.75 oz - dry measure) per gallon will give a 3% solution. If it is a liquid, then divide the above amount by concentration / 100. For example if it's 33% then divide 113 by .33 which equals 342 gr (11.4 oz). So put that amount in a container and fill with distilled water to the 1 gal mark.