It's a Aorus k7 z270x pretty sure
As you can read in specs - first PCIE x16 operates in x16 mode;
with second slot populated first and second operates in x8 mode;
third slot operates in x4 mode.
For SLI only fist and second PCIE x16 can be used. Third PCIE x16 slot doesn't support to be used in SLI configuration.