Disclaimer here, I am not an electrician by trade and I am only providing advice as a favor via this forum. If you don’t feel comfortable with this you may want to hire an electrician.
Based on what you have provided boxes 2 and 3 only have 1 romex wire coming in right? No other wires in those boxes? If so then your line and load have to be in box 1. The line will be the hot wire that you can measure 120v on using either a ground or neutral. The load is typically a wire with just a white, black, and ground, unless this is a fan where sometimes a wire with red/white/black is used. Again I say typically based on my experience in my own home.
So if you look at all the romex wires hooked up to the 4 way switch, you should have 3 total: 2 with red, white, and black that feed box 2 and 3, and another wire with just white and black. The white and black again should be your load and it may not be hooked directly to the switch, but it could be connected to one of the wires in the red/white/black romex wires. Again this wire will either be hooked up to the switch directly or one of the red/white/black wires.
Once you figure that out, then its really rather easy to setup.
Box 1:
- Wire nut the two white wires from the red/white/black romex wires along with a short wire that will go into the neutral of your new switch. This will feed a neutral to your aux switches in box 2 and 3.
- Get a short wire and connect this to your neutral bundle in the box and also hook up that to your new switch.
- Hook up both red wires to your switch for travelers.
- Put a wire nut on both of the black wires on your red/white/black wires since you no longer need those.
- Hook up the black load wire to the load
- Hook up the black line wire, 120v, to the line
Box 2-3:
- Put a wire nut on the black wire as you don’t need those.
- Hook up the white to the neutral
- Hook up the red to the traveler
I hope this helps. I about to start a project of my own and won’t be able to reply until this evening. Good luck!