One stop is no problem at all for a 3 hour enduro. We burn about 8gal/hour, and routinely make a 3 hour enduro adding just 5 gallons during our one pit stop. The only exception is Road America, where the really long straights moves that up to about 9 gallons per hour. An early tank car could still make it with 10 gallons in at the stop. Just make sure you really top off the car to start.
One caveat - make sure you've dialed in your AFM to an AFR around 13:1. A 944 will run pretty well at 12:1, or even 11.5:1 (slightly down on power), but burn a lot more gas.
Practice your pit stops. We won one enduro by 2 seconds after 3 hours, by virtue of having a pit stop 1 minute quicker (the Miata had to take on 12 gallons to make the distance, as it starts with a dinky 12.5g tank).