Are you sure about this. I'm running 3 x 16GB on a Gigabyte Z370, and CPU-Z says i'm in dual channel memory.
In CPU-Z I've also accessed SPD, and again each of the 3 DIMM's are individually reporting as dual.
There are extensions to original dual channel operation keeping it enabled in various configurations.
But it's doubtfull all DIMMs will play nicely together in every "mix and match" setup.
And would expect some latency penalties from memory controller having to balance operation with "inbalanced" channels.
Along with uneven performance varying depending what particular memory address is accessed.
(because inbalanced amount in other channel can only run as single channel memory)
Having same amount of memory/number of DIMMs per channel, with identical DIMMs in matching sockets of both channels is the surest and gives the best performance.