Hello.
The read_line_white function can be used to have the 3pi follow a white line on a black background. You should be able to use the example code with minor adjustments, such as replacing the read_line command with read_line_white.
Also, it sounds like you are a little confused about some of the properties of the sensors. The estimate returned by the read_line command is made using a weighted average of the sensor indices multiplied by 1000 (not 800), so that a value of 0 indicates that the line is directly below sensor 0, a value of 1000 indicates that the line is directly below sensor 1, 2000 indicates the line is directly below sensor 2, and so on. Your middle sensor should be sensor 2 (since the first sensor is labeled sensor 0), so a value of 2000 means that the line is directly under your middle sensor.
-Brandon