So he's mixing: 
1 gallon 2 quarts 3 pints of orange juice 
1 gallon 3 quarts 7 pints of pineapple juice and 
1 gallon 3  quarts 3 pints of ginger ale
that's: 
3 gallons 8 quarts and 13 pints. 
2 pints make a quart, so 13 pints make 6 quarts and 1 pint, so actually it's: 
3 gallons 8+6=14 quarts and 1 pint. 
4 quarts are a gallon, and 14 quarts are 3 gallons 2 quarts, so the final answer is: 
3+3=6 gallons 2 quarts and 1 pint.