Here's what your wiring actually is, based on your description. Note that I've gone ahead and color-marked the wires for their function: Black/white for supply hot/neutral, yellow for travelers, and red for switched-hot. I do that for real with colored tape; so can you.
You want to power additional loads, and they will require supply hot/neutral (black/white). You want this from the rightmost switch.
As you can plainly see, neither hot nor neutral is present there.
If you go back to the lamp, you can pick up neutral, but that's no help. You have to go all the way to the left switch to obtain hot/neutral.
I recommend running a /2 cable from the left switch. It isn't really practicable to "bring this along" in the other cable; you'd need special /4 cable to advance hot from the left switch to the lamp, and unobtanium /5 cable to bring it to the right switch along with the other circuits you do need.
Hail Mary play: smart switches
Another option is to leave the 3-wire cabling in place, but junk all the electromechanical 3-way switching and use smart switches and smart modules to control the lamp. These communicate with each other via radio waves or powerline signaling, greatly reducing the number of wires needed. However each smart switch needs power 24x7, which necessitates bringing supply hot/neutral to each switch location, including the right one. You can usually get this done within the 3 wires already in the wall; they kinda design for that.
Powering from multiple breakers
This is very dangerous. The most essential part is you must keep all neutrals matched to partner hots. So you cannot simply clump all neutrals together, you must only clump together neutrals that are served from the same "hot". This requires you be mind-bogglingly fastidious about marking every single wire in some way with tape. It's really a giant pain. I do it in THHN/conduit, but that's why I stock gray neutral wire and 10 colors of tape. If you're not into 'fastidious', don't feed the same box from 2 breakers because you will lose your mind trying to keep track of all that, and create an unsafe situation that trips GFCIs.