Modify 3-way switch to add circuit

Question

I am a DIYer with electrical wiring experience. I would like to add an existing secondary light to the 3-way circuit. The current box has two switches. The one on the left controls the main light and is part of the 3-way circuit. The one on the right controls a secondary light. I would like to remove the switch for the secondary light and have it controlled with the 3-way switch. The existing wiring is a little confusing. I have diagrammed it below as best as I can figure it out. (Ground wires are ignored.)

I would appreciate any advice you could provide. Thanks!

Answer 1

Color coding time

Cables have certain default colors of wires. We're married to that, so usually trying to "color code wires by function" is a big mess. But in this case, it works out to be very close. Let's get there.

I want you to mark both traveler wires with yellow tape (or another color, just not black, red or white). No need to distinguish one from the other. We wont be doing anything with them; I just don't want to confuse them with the wires we will be working with.

Now, remove the switch that is going away. When you do, you will free up a "pigtail wire". Remove it from the black bundle (always-hot). Tape or paint it red.

With the travelers marked, there is one red wire on the 3-way switch. Remove it and replace it with the red pigtail.

I believe at this point you have three red wires, all loose. These are switched-hot. Join them.

And we're done.