3 way switches with voltage drop
If you're measuring voltage by touching your probes to the terminals on a single 3-way switch, you're not accomplishing much.  Your readings will come out as follows.



From common to the closed traveler terminal, you'll read 0 volts. This is because common is electrically connected to this terminal, so they're at the same voltage potential.

From common to the open traveler, you'll get some arbitrary value between 0 and line voltage. This is because one of the travelers is floating, and has no reference to ground or the line.

Between the travelers, you'll again get an arbitrary value. 



From common (or the closed traveler) to the open traveler, will yield a line voltage reading, if the other switch is in the opposite position. This is because the other traveler now has a reference to ground, through the load.