For a 3 way (2 switches) or 4 way (3 switches) circuit, you need to replace both/all switches with INSTEON switches. The master switch that controls the load can be a dimmer, relay switch or keypad. The remote switches(s) can be keypad or similar switch/dimmer to the main switch.
You will then re-purpose the existing traveller wires so that only one switch controls the load and the 'remote' or 'slave' switch is simply provided with hot and neutral. Then create an INSTEON link back and forth between the switches/dimmers. This can be expanded to create 4 way or larger circuits. And by design you can also simply add another switch anywhere in the house, give it power, and link it to create 3 way switching where none previously existed.
The 3 way kit 2494M4 they mention in this video is no longer available. Simply purchase 2 or more insteon switches, dimmers or keypads.
PLEASE NOTE: Electrical work can be dangerous and is recommended to be performed by qualified individuals only. Any information provided is strictly for informational purposes.