Add the following quantities:

5 gal 3 qt + 5 gal 3 qt + 5 gal 3 qt.

First, sum the gallons and quarts separately:

- Gallons: 5 + 5 + 5 = 15 gal
- Quarts: 3 + 3 + 3 = 9 qt

Now, convert the total quarts into the largest units possible:

Since 1 gal = 4 qt, decompose 9 quarts into gallons and quarts:

- 9 qt = 2 gal and 1 qt

Combine the gallons:

15 gal + 2 gal = 17 gal

Therefore, the total is 17 gal 1 qt.

To solve the given expression, we add the gallons and quarts separately and then convert the excess quarts into gallons. The sum of 5 gal 3 qt three times equals 17 gallons 1 quart after conversion, since 1 gallon is equal to 4 quarts.

The question involves the mathematical concept of unit conversion within the topic of measurements. To find the sum of the given amounts and convert the total into the largest units possible, we follow a step-by-step process:

Add the number of gallons.
Add the number of quarts.
Convert excess quarts into gallons, if possible, because 1 gallon equals 4 quarts.

Now, let's apply these steps to the given problem:

5 gal 3 qt + 5 gal 3 qt + 5 gal 3 qt = 15 gal 9 qt

Since 1 gallon = 4 quarts, we can decompose 9 quarts as follows:

2 gallons (because 8 quarts = 2 gallons)
Plus 1 remaining quart

Therefore, the final answer is:

15 gal + 2 gal = 17 gallons 1 quart