I don't think the 3B has a fully balanced input topology. Also, If your preamp is unbalanced, then it will sound better if you use RCA to connect it to your 3B. You can experiment with 1v or 2v settings with regular RCA to see which one you like best.
If you choose to go with an XLR cables and XLR to RCA adapter, you can do it but I doubt that you will hear better sonic compared to RCA. Bryston designed the amps to sound good RCA or XLR.
However, you can experiment and post your findings here.