Make an RGB virtual mosaic from 3 single-band input files
gdalbuildvrt -separate {{path/to/rgb.vrt}} {{path/to/red.tif}} {{path/to/green.tif}} {{path/to/blue.tif}}
gdalbuildvrt -separate {{path/to/rgb.vrt}} {{path/to/red.tif}} {{path/to/green.tif}} {{path/to/blue.tif}}