In line 2 the voltage is 4.48, I was asked "when A is connected to C and A is positive, to work out the voltage of R3, the currents of R1 and R2 and the value of R3." and was given a diode value of 0.7 V.

But 4.48V applies only to line 2.

How did you determine R1 and R2?

The other question to ask is, why are all the voltages different?

 

You don't need to find R1 and R2 at all and you only need to make two measurements.

Apply a fixed voltage Vca and measure the current. Whatever that current is, that is the current that flows in R1+R2 when |Vca| is applied.

Now reverse the leads and apply the same fixed voltage Vac. You will have an increased current and whatever the increase is you know is the current flowing in R3. You also know that the voltage across R3 is (Vac - 0.7 V).

If you can't get it so that |Vca| = |Vac|, the use the first measurement to determine the R12 = R1+R2 and then use that along with the second measurement to get R3.

Using just your C-A and A-C measurements, you should get that R3 is a most likely a very nice, round value of resistance.

 

Yes, you are correct. Very observant.