Using either 3.5mm or Bluetooth output

[Nutul]

The headphones use Bluetooth 5.3, and I'd like to be able to toggle between outputs 3.5/Bluetooth.

I think you will need an external (USB...) BT device, as the Pi3 cannot - for sure - handle BT5, let alone BT5.3...

Then, for switching the output, that's just a piece of cake; so I'll let you figure it out by yourself ;-)