The weight of three gallons depends entirely on what is being measured and weighed. The most standard answer to this question, however, is that one gallon of water is 8.34 U.S. pounds, so three gallons equals 25 pounds.
The density of most liquids that we use on a daily basis is similar to water, so it is possible to estimate that the weight will be similar to that of water. However, for other substances, like sugar, the density is different. The weight of three gallons of sugar is only about 21 pounds.
In order to determine the weight of three gallons of any substance, remember that the weight is equal to the density times the volume.