Dear friends, thank you for the answears.
I made measurements using a multimeter (UNI-T UT61E, True RMS, 22k counts) on an ATX 24-pin connector connected to the motherboard.
No load, just desktop Windows:
3.3v - 3.276v
12v - 12.095v
With load, 3DMark TimeSpy Stress test:
3.3v - 3.259v
12v - 12.066v
Load - 8700k stock, 1080ti, 1 SSD, 2 HDD.
Yet it seems to me that the voltage on the 3.3v line is "undervalued"