There is probably no neutral in that switch box, and the black wire might be going to the light and is only hot depending on what position the other 3 way switch is on.
The simplest way in my opinion is to run a romex from another outlet to the location you want to add an outlet and just install the new outlet at the standard height off the floor.
If you do have an always hot black and a neutral in that box, put in a double gang box and just tap off the always hot black, the neutral and the ground.
Turn off power first.
The simplest way in my opinion is to run a romex from another outlet to the location you want to add an outlet and just install the new outlet at the standard height off the floor.
If you do have an always hot black and a neutral in that box, put in a double gang box and just tap off the always hot black, the neutral and the ground.
Turn off power first.
