Good the see that platform 3/4 does exist.
Good point. This sign had platform 3 on the right, and platform 4 on the left.
Platform codes are 3-character ASCII. So "3/4" is legit, but "9 3/4" is not. Letters are often appended to the platform numbers, not only to indicate which end of the platform is being referred to, but sometimes for other purposes. E.g. platform "11X" at York means "Come in to platform 11 on a warner route, so that the full 180m overlap beyond the exit signal is not available". Platform "17F" at Edinburgh means "17-Front" - i.e. there's another train waiting on the buffer stops, and the incoming train is double-docking on top.
One very-commonly-used platform code at *almost* every station is "BUS".