Intertwine all three of the magnolia garlands together. Just wrap them around one another until you have one really full garland.
Start adding in leaves to cover any empty areas. You can also use these to cover up the stems on the garlands themselves. Add as many or as few leaves as you like. You can hot glue these into place for a more permanent solution. Just tucking them into place works great if you are assembling this right on the wedding table.