Control switch from neighboring switch on a different circuit
A relay could be added to the circuit so that switch A, the three-way switch, controls the power on the relay's input. The relay's normally open (N/O) and common terminals can connect in place of switch B. When switch A is powered on the relay will close and this will allow current in the second circuit to flow to the lights formerly controlled by switch B.
There's a caveat: you'll need access to the switched output of the 3-way controls to make this work. You'll have to unpack the two switches out of that box to find out whether the A-switched output is available there.
Relays have three important ratings: the control voltage, the load voltage, and the load current. Load voltage and current ratings vary depending on the kind of load: motor, fluorescent lamp, tungsten lamp, etc. You'll have to search for one that meets your requirements. They can sometimes be found as a convenient UL-listed module packaged with pigtail leads like the one below.