What you measure and weigh will determine how much three gallons weigh. One gallon of water weighs 8.34 pounds, so three gallons of water weighs 25 pounds. This is the most common answer to this question.
Most liquids we use every day have the same density as water, so we can guess that their weight will be about the same as that of water. But the density of other things, like sugar, is different. About 21 pounds is all that three gallons of sugar weigh.
Remember that the weight of anything is equal to the density times the volume, so to find out how much something weighs in three gallons, multiply the density by the volume.