PCSU200: 3 different ground connections?

Instruments PC Oscilloscopes and Recorders
1

There are 3 ground connections: CH1, CH2 and OUT

Are they internally connected? (Is it enough to connect only one of them to the ground of the circuit under test?)

Or not? (Can I connect CH1 ground to “-” of power supply and CH2 ground anywhere in the circuit e.g. to measure voltage drop across a single resistor?)

Thanks a lot!

2

All grounds are internally connected together.
Also they are connected to the PC ground via the USB cable.