Wiring Leviton Smart 3-way Switch when Load Line Goes to the Fixture
No.  You cannot poach a neutral from the other switches in Box B.  This is a Code violation, potential firestarter, and will trip any AFCI or GFCI breakers in use.
There must be separation between the 3 wires coming into that 3-way and all other wires in the box.  Nothing can cross that gap except for safety ground.
Since your circuits apparently do have ground wires, you need to search the market for smart switches which do not require a neutral wire.
Since your 3-way setup is a double &quot;switch loop&quot; (i.e. both 3-ways are on a switch loop with power in the light), then you simply have no option to use a neutral-using smart switch.  It cannot be done.
The only possibility is to use a &quot;module&quot; up in the lamp proper, which talks to smart switch(es) in the switch sites.  At that point the black &amp; white from lamp to switch could be re-tasked to be always-hot and neutral.