B75MU3B does not have on-board VGA driver for Windows 10 OS.
It's due to chipset manufacture Intel did not make the driver for Windows 10 OS.
So system will use standard VGA driver from Windows 10 and cannot support S3 mode and it will automatically switch to S1 mode due to lack of proper VGA driver.
If have to use S3 standby mode, please insert a PCI-E VGA card and install Windows 10 driver for it.
Otherwise S3 standby mode cannot run properly with standard VGA driver.
