Changing Light Switch - all three wires the same color
While annoying, it is entirely legal in the USA to use all same-color wires for a 3-way switch setup.   This implies wires in conduit, where any wire color can be chosen... rather than multiconductor cable, where the factory decides the colors and they'd never put 2 reds in a cable. 

Two of the screws are the same color.  These are the messengers.   One screw is a different color.  This is the common.   If the screws are not different colors, look at the markings on the switch to figure out which is which.   

The wires enter the box in one or more cables or conduits.  The messengers will always be together.  Depending on topology, there's probably a 70% chance that the messengers exit via a different conduit than the common. The fact that the installer used the same colors makes this even more likely... if they were all in the same conduit, he'd confuse himself! So most likely the common goes into a different conduit/destination, and the installer used the same color because it was in his hand.  

Assuming we're dealing with a 3-way switch, there is no harm in trial and error; worst case the circuit won't work fully.  However, do not connect a ground wire anywhere except the ground terminal.