Current from I/O PIN

but you'd have to make sure the BJT is biased so it is controlled by its base-emitter voltage, and not the base-emitter current

So do we ensure this by rewriting physics?

A BJT is a current controlled device, you can't do anything to change that. A typical small signal BJT has a gain in the region of 200, so for 3mA you can switch up to 600mA.