The last time I did this, the channel was a plastic thing. It stuck to
the wall with adhesive backing on the plastic channel. I then laid the
wire inside the channel, and snapped the cover piece on. The fittings
for the boxes (3, in your case) go on first. Cut the channel to fit.
There is a piece to go around the corner, as well. All the stuff at
Lowe's or Home Depot. I needed to put power in a closet for a computer
printer, wireless print server, and document shredder. Maybe an hour's
work, not counting time to get the materials. Some years ago, I did
some stuff in my garage for an outlet for a trash compactor. That one
was a metal channel. Same deal, but slightly different fitting system.