I have an older house (1955) and I just bought the Meross 3 way smart switch. The smart switch has four wires coming out of it, 2 black, 1 red, and 1 white. However, my old wiring only has 3 wires, 1 black, 1 red, and 1 white. Where does the second black one on the smart switch get attached.
Is the white wire in your current configuration hooked to the switch?
Yes, it is.
If there’s no other wire in the box, you likely don’t have a neutral in the box which is required by most smart switches.
Take a look at the following thread, while not your switch, it should mostly apply.