The math
Gallons = miles ÷ MPG. Cost = gallons × price per gallon. A round trip doubles the miles, and the split divides the cost by the number of people.
With the defaults, 250 miles each way at 28 mpg and $3.50 a gallon: 500 miles, 17.9 gallons, $62.50. Split three ways, that's $20.83 each.
Which MPG to use
Your car's real highway MPG, if you know it. The EPA rating is a fair starting point. To work out your own number from a fill-up, use the MPG calculator.