It seems that your “CM3” is the same as the photo in the URL I provided, And also the one I have.
If so, there’s no point in doing anything.
There are already many reports that support this.
No matter how much you replace the “SD-Card” or Image,
The SD-Card cannot be read at boot time due to a hardware problem.
(In other words, it is not possible to boot from S"D-Card")
>> @RadxaYuntian said that first I need to flash SPI with the bootloader.
I saw this a long time ago, and I thought it was a truly bizarre claim.
This is because, The “CM3” their sell does not have “SPI-Flash” soldered in it.
How can We run “CM3” without “eMMC” variant on a genuine Pi4 carrier board ?
There is a thread discussing this, please refer to it.
(Even if it is impossible to modify the hardware,
Why we can’t boot ? You can understand the that reason.)
It is relatively easy to modify a genuine Pi4 carrier board,
But it is difficult to modify many similar products that claim to be Pi4 compatible.