On Monday, March 22, 2021 at 1:22:37 PM UTC-4, J. Clarke wrote:
On Mon, 22 Mar 2021 08:28:17 -0700, pyotr filipivich
wrote:

on Sun, 21 Mar 2021 20:22:06 -0400 typed in
rec.woodworking the following:

If you only turn off one breaker, you will often get some voltage that bleeds
back through the neutral at the supposed-to-be-dead fixture. If you used a
meter as opposed to a voltage sensor, I'll bet that it's not the full 120 VAC.

Now - although this is not something that I have ever seen - expand that to
a 4 wire source cable (3 hots and a single neutral) to the first junction box
and you've brought that 3rd breaker into the picture.

4-wire cable? It's used in dryer cables but it's a lot larger than
what you'd find in a lighting circuit.

OTOH, "I have this wire, so I don't have to go buy 'new'."

I'm pretty sure this wiring is all original to the house, which was
commercially built.

That doesn't negate OTOH, "I have this wire, so I don't have to go buy 'new'."

The original electrician may have had the 4-wire cable in his truck and used
it to save, or make up for lack of, 12/2 or 14/2 wire.

Multi-wire circuits can also be cheaper (less cable cost) and less labor (run one
14/3 cable instead of running two 14/2 cables).

As noted earlier: This is all speculation on my part.