Adding an additional fixture to an existing three way switch
You can't.  Not with steam switches.
You will need to switcheroonie over to &quot;smart switches&quot; for this to work.
You will need smart switches that communicate either wirelessly or via powerline signaling. You won't have ANY spare traveler wires.
On the inter-switch 3-way cable, you will reassign all 3 wires to new jobs, which will be obvious.

White = neutral (smart switch and lamp need this)
Black = always-hot (hot at all times, the smart switch needs this)
Red = switched-hot (hot when the lamp is on)

Both lamp hots get attached to the switched-hot wire.
Both lamp neutrals get attached to neutral obviously.
One of the smart switches attaches to always-hot, switched-hot and neutral.
The other smart-switch attaches to always-hot and neutral.
Often the layout of the 3-way circuit dictates where the master must be - but not this time. You can have the master in either location here.