RGB compositions are very important in remote sensing studies, however, there doesn’t seem to be a function in QGIS. But there is an easy trick by creating a simple text file which stores the paths to images.
So go to “Raster – Miscellaneous – Build Virtual Raster (Catalog)”
pick the 3 raster layers, tick “Separate” and define an output file. That’s it. Now the text file can be loaded and the RGB displayed, like this RapidEye image with NDVI as red and bands 3 and 2 as green and blue.