can anyone help me design a 3-bit down counter that counts from 7 to 0? and when the counter reaches 0 an Led should light up to indicate that it reached 0. The inputs consist of Start/Stop pulse switch, a Preset pulse switch and a Clock Input. The output should be connected to the LEDs.

 

I suggest you take a read here:
http://www.allaboutcircuits.com/vol_4/chpt_11/2.html
http://www.allaboutcircuits.com/vol_4/chpt_11/3.html

and come back with your questions.

 

damn i dun understnd

 

I believe there is enough informations in the links above. Anyway Counting down rather than up simply requires cascading the clocks using the inverted outputs.
Check the attached description, it's an MOD-8 asynchronous down counter that counts from 7 to 0:

 

You can use an off-the-shelf down-counter or you can build it with flip-flops using Karnaugh maps.