How to check if RAM is working in dual-channel mode?
Solution 1:
First, your mainboard must support flex-memory to have dual-channel support for 3 memory modules if it doesn't have triple channel mode
But even if flex-memory is supported then it won't work in dual-channel mode when you install 3 RAM modules with the same size. Have a look at Intel's document, the only way that work is that one module's size must be the sum of the remaining two.
Dual-channel with three DIMMs
Unfortunately it seems AMD doesn't have anything similar to Intel's flex memory