Hey, I got a little confused and I wish I can get answer! Now, if i have a circuit with 3 diodes (9V, 7V, and 5V) is that true one of them must be on which is 9V and the other have to be OFF?

Thnx

 

My crystal ball is in for a tune-up, so I'm also confused.
Can't answer your question without a circuit diagram to see how you connected them.

 

 

 

Diff. Voltages mean zener diodes i believe.

If they are in series AND rev. biased....they add up to create a total required voltage of 9+7+5 for them to switch 'ON'. They will all be on for current to flow....vis a vis Kirchoff.

If they are all fwd biased, in series, they will all be on with a small voltage supplied. Much smaller than 5V, perhaps 1.8V.

If the are in parallel and again rev. biased, the lowest value will switch on first clamping the voltage and preventing the others from conducting.
Being zener diodes your circuit WILL require a current limiting resistor to perhaps limit the current to a few mA, else you can burn the parts.

In fwd biased parallel (and with a current limiting resistor) the voltage across them will be clamped to around 0.6V...a single silicon PN junction Vf.