Take Alice and Bob passing each other in space, in inertial relative movement.
Alice can consider herself at rest, and Bob is passing at 100 kph.
Bob can consider himself at rest, and Alice is passing at 100 kph.
Alice has a clock that ticks at 1 second per second, but for her, Bob's clock is slow.
Bob has a clock that ticks at 1 second per second, but for him, Alice's clock is slow.
Thus, their clocks cannot be synchronised. This is basic relativity.
Well, yes, you can insert Carol who remains between Alice and Bob, for whom they are both doing 50 kph. For Carol, Alice and Bob's clocks tick at the same rate.
Your scenario seems to have the clocks meet at a common event, ...
No, I don't think it does.
... but to generalize a bit, and to remove all unnecessary observers, consider flat spacetime containing two inertial clocks at arbitrary locations, moving at arbitrary velocities, and set to arbitrary times.
In exactly one frame C will those clocks be moving in equal and opposite velocities. ...
Yes, the simple scenario implies this. Though I named two velocities to try to make it less abstract for the OP.
(Personally, I think the OP is diving off into Twins' Paradox etc, with ever increasing complexity, when they don't really grasp the significance of the basics of relativity. I think the explanations need to get simpler, not more complex.)
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