It has been suggested that: :You have to run a new wire to the other 3 way switch. use 14-3wire for a 15 amp breaker and 12-3 for a 20 amp breaker. use the black for hot, white for neutral and red for your identifier. hook ground to switch and box if metal. The red tells the other one what position the other switch is in. Of course your switches have to be changed to 3 ways. If you are not ABSOLUTELY clear on this, seek help because a fire can result! This may be at least partly accurate, however it does not completely cover the situation. It is unclear why you would ever have two blacks but only one white wire and where they might lead, and what exactly you're trying to do. You could be adding danger to confusion unless you figure out exactly which wire does what. Also, there is often no neutral needed between 3-way switches and all three wires are hot at one time or another (two travelers and one switched). Any white wire used in 3-way (traveler or switched) must have its insulation taped over (at both ends) with black or red to show it is no longer to be considered neutral. The only time you would have neutral between two 3-ways is if the light is between the two switches rather than the switches being connected directly to each other.