https://conduition.io/cryptography/frost-byok/
Maybe something here.
Is it possible to take an nsec you already have and split it into 3 somehow, give one shard to each server and then they somehow collaborate to make a signature? I think you can't do MuSig2 for that, but maybe some other, simpler (less safe?) algorithm? I've asked nostr:npub1vadcfln4ugt2h9ruwsuwu5vu5am4xaka7pw6m7axy79aqyhp6u5q9knuu7 this once but he never answered.