You should be able to just use http://fonts.googleapis.com/css?family=Rosario:400,700,400italic
which is the “combined” URL given by Google when selecting multiple weights/styles of a font. You can then just register the font once. You don’t need to over-complicate the $handle
either, something like google-fonts-rosario
should do just fine as long as it is unique as mentioned in the codex.