PixInsight and ImagesPlus store multichannel FITS images in different ways.
PixInsight stores a multichannel image as a data cube: a single image with three dimensions, where the Z axis represents the channel index.
ImagesPlus stores each channel of a multichannel image as a separate image in a FITS file, each one with its own HDU (header-data unit). Hence, when you open such a file in PixInsight you get just what has been stored in the file: three images in the case of an RGB color image.