How to add a light fixture to an existing 3-way circuit?
You can't just pick any random point and fork a lamp off that.  They do not work that way.  

The simple way

3-way circuits have several tricks and traps, and you can't get anywhere with them unless you really understand how the circuit functions, how switches are designed, and I really, really recommend wires be marked by function with colored tape. 

So the simple way to do an end-run around all that complexity is supply the second lamp from the first lamp's location.  You are simply grabbing the 2 wires (and ground) supplying the first lamp, and paralleling it to a second lamp. Badabing, done.  You never have to think about 3-ways. 

The hard way

You can stop and train up on wiring and 3-way circuits.  I'm all for education. But in this case, this will only make you realize you can't get there from here.  You cannot feed a second lamp from this location. 

The smart way

This is a different kind of smart, where we toss traditional 3-way wiring into the dumpster of history, and go with "Smart Switches".  There is a huge variety of smart switches, and we're not a "Shop For Me" site... but there's at least one out there that'll Git-r-Done for you.  

What's unique about smart switches is we toss out the goofy schemes and oddball wire functions found in traditional 3-way wiring.  We reduce the wires to 3-4 functions: 


Always-hot goes to every switch location, and use Black. 
Neutral goes to every switch location and every lamp, and use White. 
Switched-hot goes from the "master" smart switch to the lamps, using whichever wire color is left. 
Some smart switches require a datacomm signal between switches, that'd most likely be red.  Red can't do 2 things at once. 


Ground is a safety shield and can't be used for any of these.