The splitboard I am getting is 292mm (nose...
The splitboard I am getting is 292mm (nose width), 252mm(waist width), and 290mm(tail width). Will the 140mm wide skins be alright?
Yes. The 140mm refers to half the pair. I'm guessing that your 292mm width is the board in downhill mode and that a half pair is 146-126-146. You will need to trim the outside edge of the skin to match your boards sidecut. G3 includes an awesome trim tool with the skins.