I have uploaded a rough schematic, hope you would have an idea what i 'm making....
IC = LM324
suggest any other schematic that can be used using single pot!
There are a couple of issues with your circuit.
First off, you shouldn't short circuit the outputs of an LM324, it is a quad op-amp and it has got push pull outputs. The LM339 is a quad comparator and it has got open collector outputs, which can be shorted together without problems. The LM339 outputs needs a pull up resistor as well.
Second, you are shorting all the non-inverting inputs, so if one or more of the photo transistors gives enough current through, it will affect all the op-amps, as they won't function as indepentant amps coupled like that.
The negative feedback (giving hysteresis) will affect all amps as well.
Hysteresis is the effect where you change the shifting point of the circuit, by leading a part of the output signal to the inverting input. With a hysteresis of eg. 100 mV and a shifting point of 2V going upwards will then change the shifting point to 1.9V that it must go under to switch back to the 2V. This is done to avoid an unstable situation where it oscillates around the shifting point.
The circuit you have (seen for just a single amp) is a bit strange, as with the output low, will have a shifting point of 0V and when the output is high, the point will be ~1/3 Vcc.
I don't know why you shorted the outputs together, since the point in having more than one detector is to be able to tell which of them see a line, but that takes separate outputs fed to the rest of the circuitry used for the 'bot.
To change the circuit into something useable, you need a bit more resistors, to break it into separate inputs, outputs and feedback.
Using a single pot, it would be easier to place it in the inverting inputs circuit.
What will the outputs go to?