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.
Hope this helps.