3 different private key-looking strings
One is 32 characters, starts with HDr…
Two are 64 characters, start with oUC… and MHC…
None of those resemble any format of a private key;
The 32 character's length and characters isn't recognizable.
The 64 character strings don't look like ECDSA private key for having non-hexadecimal characters.
Or have you replaced the the starting characters with something random?
Questions:
1. Could these be the same private key in different formats (e.g., hex, WIF, Base58)?
2. What’s the best way to decode and test which key is valid for the address?
3. Can Electrum or another tool help me test them safely offline?
1/2: Check this thread for the formats and possible ways to recover:
/index.php?topic=4959742.03: No, it should be in the specific formats that's supported by Electrum: WIF or Mini-Private key.
This was back in 2010 when everything was non standard. He likely used cli tools to generate the key. Is it possible that they may be custom encoded?
I don't know how to attach an image here.