There's no need to pay for a pricey electrician for a light switch problem— you can try to fix it yourself. This video from This Old House explains how three-way switches work and shows tips for finding and replacing faulty three-way switches in your home. First, you need to understand the difference between the hot and neutral wire, the ground wire, and the traveler.
1 Start by turning off the power to the circuit you'll be working on at the main electrical panel.
2 Test the switches with a voltage tester to ensure the power is off.
3 Remove the cover plate from the switch box with a slotted screwdriver.
4 Unscrew the three-way switch you suspect isn't operating properly from the box.
5 Before disconnecting the switch wires, label each wire for easy reattachment later, and mark the "hot" white wire with a black marker.
6 Remove the suspect switch from the box and use a continuity tester to test its operation. If you find that current is allowed to pass through the switch, regardless of the position of the on/off toggle, then replace the old switch with a new three-way switch.
7 After tightening the wire connections to the new switch, wrap electrical tape around the switch.
8 Screw the switch to the electrical box, and replace the cover plate.
9 Test the new switch by flipping on and off the three-way switches at each end of the circuit.