I'm replacing dumb switches with the GE Z-wave Dimmer and the add on. Total of two switches going to a light fixture.
For the dumb switches it's the easiest setup ever. No neutrals tied together in the boxes, just hook to coinciding screw in both boxes and everything works. However, I cannot get it to work with the Smart Switches.
If you don't mind taking a different direction, you can save the GE switches for a different application and use a relay switch module in the light fixture - something like the Enerwave ZWNRSM1s. It simply hooks up to the source wires in your fixture and then tucks into the electrical box. The black hot wire going to the light would connect to the trigger and the relay's load wire would connect to the light. Every time the trigger changes state, the output will toggle, so if the light is on, flipping either switch will turn it off and if the light is off, either switch will turn it on.
I haven't tried this with a dimmer module, but it should work in a similar way. Enerwave doesn't seem to have a dimmer version, but Fibaro does (Fibaro Dimmer). I'm not sure how the dimming function would work using standard 3-way switches - maybe you would have to change the switches to momentary ones or the dimming would only be done remotely using ST.
Just an idea if you can't make what you have work without running neutral wires to your switch boxes.
The issue with this setup is you need 4 wires at the main switch: line, load, neutral, and traveler. You only have 3. So even changing wiring in the fixture won't help. If there is another switch in the same gang box using the same line from the breaker then some have had success using a line or neutral from that wire.
As @DaveR12 mentions a relay is really your only option. Aeon makes them as well.
I would also suggest a connected relay in your fixture box. I use an aeon micro switch with wiring similar to yours, I can control the light from the ST app, with an automation or at both wall switches.
Alternatively, if you use incandescent or halogen bulbs, there are older model connected wall switches by GE and Cooper that don't require neutrals. But I don't know how easy they are to find anymore.
I kinda figured that was gonna be the case. I didn't realize there were relays like that or I would have ordered a bunch of them. Thanks for the quick responses. Next time I'll ask first before I spend hours staring into the wall trying to figure out how to make another wire magically appear to make it work. One with a good dimmer would be ideal. I'm kinda getting used to the soft fade in/out from the GE Dimmers. First world problems. . . .
The Fibaro Dimmer 2 does work in the configuration you are suggesting, but not all in-wall modules do. So just be careful. The Aeon Micro Dimmers will fry if you attach 120v to their switch terminals.
If you want to Dim from the wall switch then you would need momentary switches. Or, you can dim through SmartThings/Google Home/Alexa, and just use the light switches for on/off. A cool feature of the Fibaro is that you can set a default dim level when you turn the device on from the wall switch (say 30%) and then enable a double toggle to turn it up to 100%. Something like that usually covers the two dim levels I use most frequently.
Purchasing a relay is the simplest option but as a note you could also use Cooper or Linear 3-way switches with that wiring setup. Their add-on switches have a Z-Wave radio so a traveler is not needed. The Linear switches look identical to the GE switches except they have green LEDs.
Here is another option. If your don't plan to have LED lights you could go with the RF9534 or RF9536 and its associated accessory switch RF9542. These don't require neutrals and were designed to be used in your situation. The neutral at the switch is necessary if you plan on using dimmable LED so if incandescent is ok with you than this switch will work.
I have a track lighting with 3 pendants and 6 LED spots which I don't believe are dimmable. At this point I want it all done the same so Telling my Google Home to Turn Off the lights and watching them all fade out at the same time is pretty awesome. Even if no one else cares. So I might have to go with the Relay. Heck at this point, maybe I'll just switch out the LED spots, they aren't quite as bright as I'd like but since it's on a middle floor, I don't feel like ripping out drywall and redoing for recessed lighting like I want.
OK, I was making an assumption that you wanted dimming light control for those lights. The part I put up earlier was for dimming. If you want dimming the Eaton switch will give you what you want with full dimming at both switch locations but the only downside is you will need to put in incandescent bulbs to give you dimming capability. It doesn't hurt to try to dim your existing bulbs but they might flicker on you as you dim down off of 100%.
The relay method will work fine for you as well but it will only be on-off control. In addition to Fibaro Dimming another dimming relay type option for you is the new Nano Dimmer by Aeotec that doesn't need neutrals. You would mount it up at the ceiling and wire your two switches back into the Nano.
So the simple reason this is called power through light is because the line from breaker first enters the light box. All regular switches 2+way pass a line in and load back to the light, but the wiring to accomplish this differs based on the origin of the line and exit path of load.
Smart switches on the other hand also need access to neutral, so when line (and neutral) enters the light box, you can't normally use them. In the diagram above you have 3 wires to the switches, since you also need neutral you would need to have a way to pass 4 wires (neutral, line, traveler and load) with standard GE switches.
Here is a thread about GE 3-way switches and what can and can't be wired with existing wires.