Probe the existing hole with a keyhole saw to find the horizontal edges of the lath. Heres how to do it without breaking the plaster and lath.

How To Replace A Light Switch With A Switch Outlet Combo
Old light switch has 3 wires. A 3 wire nm connects the traveler terminals of the first and second 3 way switch together. The white neutral wires are connected together in each switch box. The third black wire is not twisted together. The necessary supplies though are critical so make. Changing a 3 way switch to a wifi smart switch can be tricky. Light switches have 3 black wires and 1 grounding wire coming to the box.
The white wires are capped togather read more. The most basic three way switch setup has two switches that connect and control one light. 3 wire lighting systems the 3 wire system is less common than the 2 wire system it includes a neutral at the switch as well as the live and switched live wires. Diagrams shown on this page are simplified for clarity. If you have a 2 gang or 3 gang switch you may have black and grey wires however these will have a brown sleeve on them as they are also switched live wires. How to replace your old outdated 3 way light switches step 1.
That is why it is important to identify the function of the wires for 3 way electrical light switches. Fix old house wiring problems. Two of the black wires are twisted together. Traveler wires are interchangeable on each switch. The common terminal of the second 3 way switch connects to the light fixtures. There are old existing 3 way electrical switch installations that do not follow acceptable industry practice or electrical code requirements.
You wont need many tools for this job. Bring old light fixtures wired with knob and tube wiring up to code by installing an electrical box in the plaster wall. Light switches have 3 black wires and 1 grounding wire coming.