If one gallon equals 3.78 litres, how many litres are there in three gallons?
So you have this ratio of gallons to liters:
Multiply the number of gallons by 3 to get 3 gallons, and to keep the same ratio, you must also multiply the liters by 3 as well.
Using ratio in fraction format. Note that this is not a fraction but a direct comparison of counts.
But we need 3 gallons so we need to change the 1 into the value of 3.
Multiply by 1 and you do not change the value. However, 1 comes in many forms.