If the same nonce k (same R value) was reused across different private keys, and you already know the private keys for A and C, there’s a real chance you can recover k from their signatures. Once k is known, you can use it to solve for the missing private key B using its signature.

The GCD of 63 between A and C’s keys suggests they may have been generated using a weak or biased RNG, which could make things even more predictable.

So yes - if k is reused and recoverable, this setup could absolutely reduce entropy and potentially reveal dB. Definitely worth deeper analysis.