3 GPU working and all 6 gpu detected.

"Free Resources" is the key, which is what the OS is telling you, so likely is an OS issue.

We see this problem in the forum occasionally. One of the best diagnosis was the system memory that Windows sets aside for its use when working with hardware, which has a limited size, is used up by all the video cards once you get past a certain number, three, four, whatever.

The BIOS of some (older) boards have an option called MMIO, Memory Mapped IO, which let the user adjust the size of that memory area, or make it dynamic, meaning growing as large as needed, within some limits. The user was able to solve the problem by setting this option to dynamic, IIRC.

Newer boards tend to not have this option. Yours doesn't, and none of mine do. That may be related to the newer versions of Windows keeping control over that itself at all times.

When you have this problem (all the time now, right?), in Windows, open Resource Monitor and click on the Memory tab. You may find a hint about what it going on.

Also, open a Command Prompt window, and enter msinfo32 and press Enter. You should get a System Information window. Start in the Hardware Resources section, and go through each category, watching the entry in the Status column. In the Components section, check the Problem Devices entries.

When you keep adding or changing video cards, you should be clearing the UEFI/BIOS every time you do that. That gives the BIOS and Windows the chance to reallocate resources for hardware, and might help your situation. Not doing this could be at least partially why you get the weird results with only so many cards working when more or fewer are installed. Changing their locations in the PCIe slots also can cause confusion that a UEFI/BIOS clear can help.

Did you try asking about this in a video card manufacture's forum? They no doubt get more questions about this than a mother board forum.