Two light switches into one light
Sounds like you might be out of luck, unless there happens to be a neutral in the box.  When it comes to 3-way circuits, a white wire is likely not a neutral (and in your case I can say pretty certainly that it is not).

A 3-way switch has four terminals, none of which is neutral (unless it's a &quot;smart&quot; switch).
Common
This terminal is either connected to the line (power feeding in), or the load (feeding power to the device).
Traveller
There are two traveller terminals, which the switch selects between depending on the position.  If power is fed to the common terminal, power will be directed to one of the travellers depending on the switch position.
Ground
This terminal provides a connection to the grounding conductor.