Understanding current flow in example
Assuming that the three light bulbs have identical characteristics, the voltage in their terminals will be exactly the same for each one. In this way, the sum of the voltages of the three bulbs will be equal to the voltage of the source, as follows 

Vs = Vb1 + Vb2 + Vb3

where: Vs = Voltage source (battery)
       Vb1...3 = Voltage of bulb 1...3

The difference in voltages when measuring (e.g. with a voltmeter) depends on the terminal that you use for reference (negative, ground reference). If you measure with respect to the source's negative terminal (typical measurements), the bulb closer to the positive terminal will measure the maximum voltage, then the second and the lower voltage for the third.

The bottom line is that when measuring the reading is a sum of the voltages across the device's terminals, but the voltage in each device must be exactly the same.