Intel has confirmed the existence of Z390 PCH
The Z390 and the rest of 300-series chipsets will support Coffee Lake (CFL) and Cannon Lake (CNL) CPUs, according to Intel document.
The document also revealed Cannon Lake and Coffee Lake series (HEDT) designed for X399 chipset. Intel was expected to skip this codename as it was already taken by AMD for the first-generation Threadripper.
The file would, therefore, suggest that both Z390 and X399 are coming soon, likely around Computex, as we previously speculated.