As others have pointed out, you have a huge menu, and no matter how you style it you're probably losing customers who don't want the hassle of reading through 20 menu items to work out which one is best suited for them.
Assuming you have to have a 3 level menu though:
On mobile you can have a Menu link which opens up a scrollable menu list. When an option is clicked, you slide that set of menu options off the screen to the left and show the sub menu options in another scrollable list. You can repeat the same for a third level. Scrolling lists like this is very common on mobile, and this experience is probably better and more familiar than any desktop experience you can create.
On desktop, 3 level navigation is uncommon. If sites like Amazon, eBay, Microsoft and Apple can create sites with only 2 levels of navigation, you have to ask yourself why can't you. I would keep "Administrative and Public law" as it is (as a link to a new page) and include the children of that on the new page. When you click "Mac" on Apple.com, you're led to a new page and presented with what are essentially sub-categories but as large thumbnail images and text at the top. This is easier and less fiddly to navigate than hovering over menu items and then sub menu items:
If you really need to do 3 levels of navigation on desktop though, you could do something like Amazon, where you have multiple instances of their 'Shop by department' for 'Home, Personal legal Services, Business Legal Services, Our People, etc'. So, click a main menu item and see a single column list with small line-height for quick scanning, then hover over one of those to explode another sub-menu on the right.