There are 2 ways a 3-way might be connected with K&T. In both, 2 wires go between the 3-way switches. And at one end the 3rd wire goes to the light socket connection to the end of the light bulbs.
================== In one way of wiring, the 3rd wire at the other switch goes to the other (screwshell) terminal at the light sockets. This is not a good way to wire a light.
================== The other (more likely and preferred) way of wiring a 3-way, the other wire at the 2nd switch does not connect to the light socket. **This will be the hot feed to trace.
(With both wiring methods the 2 wires between the switches connect to the same-colored terminals on the 3-way switches.)
====================== Don't know if you noticed - Terry started a 2nd thread on this 2-1.