maybe someone can help me... I want to measure 3 different resistors with the Arduino Mega.How it works with measurement and a voltage divider I know and that works fine if I want to measure only one resistor. With 3 resistors .. then I have a parallel circuit and it can no longer work. I also have only 1 voltage source available.
Does anyone know how I can build the circuit?
OK, maybe I'm missing something, but that should work just fine. Obviously you'd need the three analog ports as you have shown, but what is the problem?
Each vertical pair of resistors has 5V at the top and GND at the bottom, so each will work as a voltage divider, just the same as a single resistor pair.
The only thing you can't do is work out the resistors by measuring the current drawn from the 5V supply. But you aren't doing that, so there should be no problem.
Or have I totally misunderstood what you are asking?
The measured values with several result in total nonsense because R1, R2, R3 are connected in parallel.
In the parallel circuit, the resistors cannot simply be added. With each additional resistance, the total resistance of the parallel circuit is reduced. This is easy to explain because with each additional parallel resistor, the current has another branch to which it can be distributed.
To find the total resistance, the conductances of the individual resistors must be added together. (The conductance is the reciprocal of the resistance, so 1 divided by the resistance).
I know all about resistors in parallel and serial. 
The key point is that you are trying to find the values of Rx (three of them, so maybe you should call them Rx, Ry and Rz). You are doing this by measuring the voltage at the midpoints, where you have labelled A0, A1, A2. Correct?
If so, then it WILL work, because each voltage divider pair is connected between 5V and GND, so the voltage at points A0, A1 and A2 must be in proportion to Rx / (R4 + Rx), regardless of how many other voltage divider circuits you have also connected to the 5V and GND rails.
Another way of looking at it. Those three resistor chains are connected in parallel across the 5V and GND rails. So if you want to measure the overall resistance of the entire circuit, then of course you must use the 1/Rg = 1/R1 + 1/R2 + 1/R3 formula.
But, you are NOT trying to measure the overall resistance! You are simply trying to measure the voltages at each of the three points A0, A1 and A2. And those voltages are set by the following formula:
V(A0) = 5V * (Rx / (R4 + Rx))
It cannot be any other way - there is 5V at the top, GND at the bottom of each resistor pair. Does that make sense now?
If not, find yourself an online circuit simulator and try it. 
You can put the resistors in series then sample at each point two resistors connect to each other. Each point will give you a different voltage. The resistors do not have to be the same however you can calculate the voltages at each point.
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