Layer stacking rasters in QGIS?
I had the same issue some months ago.
Use gdal_merge to generate a new file from the 3 independent rasters.
In OSGeo4W command line you can do this:
gdal_merge.bat -separate -of GTiff -o output.tif input1.tif input2.tif input3.tif
In QGIS you can do the same with a GUI in the raster plugin "merge" tool.
You could try "Raster -> Miscellaneous -> Merge" which is a front-end for GDAL's merge_raster.py. It's part of GdalTools plugin that you may have to enable from the "Plugins -> Manage plugins..." dialogue.
Geoprocessing was moved to processing toolbar in QGIS 2.16, as you can see below: