Wordpress Mega Menu

I require the Wordpress menu to be set up so for a desktop device, the 2nd level menu is 3 columns (with text centred in each column with appropriate padding each side), and the 3rd level menu to follow the same layout.

There should be no overlap with the main nav (i.e. 2nd and 3rd nav should appear below the main nav). All styling should be to a high standard.

The current menu was worked with another PPH freelancer but it wasn't satisfactory, so please only submit if you're able to complete this as described. A backup of the existing menu would be preferable.

Current website: https://tinyurl.com/yrnuxfub