Fiji image size limits on composite images

I am trying to generate an extremely large composite image. I have red green and blue fluorescence images as 8 bit tiffs which are about 40,000x40,000 pixels, each image is about 1.6 GB. In fiji can merge them as separate channels and then generate a single 3 channel tiff with the 3 images in them but if I try to convert this into a composite image it does some of the image fine but corrupts segments of it with what looks like the original single channel monochrome images. Any suggestions about how to merge these images into a single 24bit tiff?

Thanks,

Ian