Remember that when you are making stimates, you are going to roiund to the nearest dollar. Since $3.29 is closer to 3 dollars than four dollars, Sam should use the 3 dollars figure to estimate if he can afford 4 gallons of gas.
$3+$3+$3+$3=$12
The actual value is

, so $3 per gallon is indeed a god estimate.
We can conclude that the correct answer is:
$3 + $3 + $3 + $3 = 12, so Sal can afford the gas.