How can I create a new group for items in the Sidebar in Finder?
No. You can rearrange the order of the three you mentioned though - just click and drag to move them. 

A possible workaround would be to create a new folder, say "Cloud files", and put it in the sidebar. You could then place aliases of your other folders into that one. This would help trim down the folders in your Favorites section, as well as keep the same quick access to them.