Need help figuring out X11 3-way switch connections (6 wires from one box)
My guess is (and this is only a guess pending more information):

The 3 wire cable containing 'A' goes to the light(s).
'A's associated black wire is unused and its white wire 'B' provides neutral for the light(s).
The travelers' associated black wire is wirenuted to the feed wire and connects to the other 3-way sw (you labeled it 'C') on its common screw.
The travelers and 'A' connect to the 3-way sw with 'A' on the common screw.
The wires 'B' and Neutral wirenut together to provide neutral to the light(s).
And finally because a conventional sw doesn't need power, get rid of the pigtail you labeled as 'Hot', while keeping the 2 black wires wirenuted together.

The more information I mentioned is this:
It looks like there're 2 3-wire cables coming into the top of the box and 1 2-wire cable coming into the bottom of the box. label the wires, including that black wire that looks like it is shoved into the box unused, as to which cable they're associated with, e.g. cable #1, cable #2 and cable #3. Because cables and wires have two ends, knowing where the other end of a wire is can tell you more than what a tester or meter can.