Touch one meter lead to what you think is the "commom" touch the "second" meter lead to one of the other screws. If the meter reads close to zero you have identified a possible "traveler".
Keeping the leads in-place, flip the switch. If the reading goes to "infinity" (like nothings connected), move the "second" meter lead to the other un-known screw.
If the meter now reads zero you have identified both the "travelers" and the common.
"Travelers" are wires (usually black and red) that connect 3W switches. It does not matter which color lands on each switch but they must not be landed on the "common" screw.
Any number of 4W switches can be inserted between 3W switches. Put the red & black travelers from the first 3W switch on one end of the 4W and the red & black from the other 3W switch to the other end of the 4W switch.
A wiring diagram is usually included with each 3W or 4W switch.
3W & 4W switches are usually wired with 3 wire romex with a ground. The wire will have a black, red and a white.
All grounds found in a box or fixture should be tied together. They will usually be bare or green.
The statements above will always be true but, depending on where power is applied in a circuit (in the ceiling fixture or in one end of a 3W circuit)the wiring diagrams will vary slightly.