It seems, that new version o Orange Pi 3B has landed, now with version 2.1:
The main changes are
Is this new version supported in dietpi? Specifically, does BT work? Are there drivers available for this new chip?
They support the original DietPi - Lightweight justice for your SBC!
according to the download website. I would assume this is supported probably. Do you currently own one or just looking before buying? I would say just test it if you had it on hand
I dont have any in my hands, but want to buy it. But im not good at hardware stuff so im affraid if included drivers will not work i will have chip without BT. For my use case BT is crucial. Maybe i should take the risk 
The hardware is supported. At worst case you add a USB Bluetooth adapter too.
Sadly no one is gonna have all the answers for you.
You gotta dig in and figure it out.
Once you have the hardware and dietpi OS installed then I’m sure someone in here can point you in the right direction to setup Bluetooth