Is it possible to connect the 3.3V, 5V and 12V pins of the system power supply in series to increase the output voltage?

Question

Power Supply

Label power supply: 3.3V 26A, 5V 28A, 12V26A.

Now I only take 12V and 20A (240W) from it, but I would like 20.3V with the same current.

Or will it be like taking 2 wires from each end of the battery and connecting them in series in the hope of doubling the voltage, in fact shorting it out?

Answer 1

Or will it be like taking 2 wires from each end of the battery and connecting them in series in the hope of doubling the voltage, in fact shorting it out?

Exactly.
Look at the schematic.
If you want 20V to 20A, then just take a more powerful PSU and a boost converter.