Installing a New Electrical Outlet in a Modified Closet?
On a 3 gang box with 12 AWG wire I would not use a plastic old work box the tabs are not large enough. I would use a metal box with wings the wings expand among each side, they come in single gang that 1 side can be removed off each end box and both sides on the center box then they attach to each other. Screwing to a stud as Harper suggest is also a good idea. I have no problem installing outlets in a closets I have added some for things like dehumidifiers many times. The lighting you are going to need to be careful with because this is a closet look at NEC 410.16.C for the type you can use in a closet and the allowable locations.