3-way, generally means that you can control a light from 2 locations. The 6696 offers that ability.
Your scenario is slightly different in that you presently have 2 mechanical 3-way switches and wish to add a 3rd location.
If all three locations are within 50 feet, the devices should not be installed in metal boxes and the lighting must be incandescent, the 6696 may work for you.
First, identify the 3-way switch that has the load wire (wire that carries power to the light), replace that with the line voltage switch from the 6696 pair.
Use the battery operated remote for the 2 remaining locations, set all devices to the same letter ("A" or "B").
To do this, you would have to purchase 2 kits as units are only sold in pairs.