Are you using UberMenu 3? Then you’ll want to use Columns and set Submenu Column Default for your header item to divide them into the appropriate number of sub columns
If you want to split up a set of third level submenu items into multiple columns under a second-level header, like this:
You can keep all of your menu items under that header like this
And begin a new column by checking the Start a new column (under same header) option on the item that should begin the second column.