Hi apranax
I presume you are using a wheeled robot perhaps 2 wheels and 1 castor wheel and that the sensors are mounted on the front and that you are trying to follow a black line on a white surface. Given this, here is the logic(no code, you'll have to do some hard work)
Sense all the three inputs from the sensor(left, middle, right) using analog read. now whichever sensor is on the black surface, will give a low and the others an high. So you will have nine possible combinations. First workout the actions for these combinations(there are four actions possible - go straight, stop, go left and go right). Run the combinations through a switch case and perform the actions.
performing the actions would mean controlling the 2 dc motors. any 1 of these motors running forward with the other running backwards will produce a turn in the direction of the motor running forward. so wire up the dc motors to the h-bridge and send output to the h-bridge from 4 digital pins on the arduino. things to remember - you would need to provide a separate power source(depending upon you motor requirements) for the h-bridge with a common ground to the arduino. intially you might not want to drive their motors at their full voltage ratings(they might be too fast and you might miss the line!)
Hope you can do the coding with this.
Cheers,
pracas