If you install 3 RAM modules with the same size then most probably it won't work in dual-channel mode. Have a look at Intel's document, the only way that 3 modules work in dual channel mode is that one module's size must be the sum of the remaining ones. Otherwise it must be in triple channel mode, if your mainboard supports it
Dual-channel with three DIMMs
Unfortunately it seems AMD doesn't have anything similar to Intel's flex memory