Part 1 -
The voltage between the power and the ground rows on the breadboard was approximately 4.92V, with about 12V coming from the adapter socket.
Part 2 -
The voltage across the first resistor was approximately 3V. The voltage across the LED was approximately 1.9V. These do have a sum that is approximately the total voltage in the circuit.
Part 3 -
The voltage across the first LED was approximately 2.52V and the voltage across the second LED was approximately 2.48V. These are nearly the same, as expected. Three LEDs in series do light, but they are less bright because each receives only approximately 1.6V.
Part 4 -
In parallel, the LEDs each have the total voltage of approximately 4.9V (4.94V, 4.94V, and 4.92V were measured). The amperage drawn by the LEDs was approximately 20 milliamps.
Part 5 -
The potentiometer drew between 0V and 2.98V, depending on the setting.