Is 3 liters bigger than a gallon?

3 liters is less than a gallon.

1 US gallons = 3.785 liters

1 Imperial gallons = 4.546 liters

One liter is about 32 oz. A standard two liter drink bottle is 64 ounces. Three liters is 3 times 32oz, or 96 oz. A gallon has 128oz (4 liters), so to answer your question, no. Three liters are not more than a gallon in terms of volume, they are 1/4 less.