2.from your experience, does it really upgrade the user's experience?
All the folks who have been tracking this sort of thing (more below) are convinced it hurts user experience.
Your current three-level navigation is troublesome enough. Please don't make it any harder on your users!
I only got to the 3rd-level menu on the third try. Because I don't have a perfectly stable hand, mouse and surface, my mouse movement wasn't smooth enough, and kept jumping to the wrong place when I tried decending to the next level.
Sure, that may be just me and the few of us "disabled" people.
But that's the least of the problems with multi-level navigation though.
Even for perfectly abled and perfectly equipped people, testing after testing shows multi-level navigation present significant usability and accessibility challenges.
From Nielson Norman Group
(making the case against mult-level navigation in favor of mega menus):
For bigger sites with many features, regular dropdown menus typically hide most of the user's options. Yes, you can scroll, but (a) it's a pain, and (b) scrolling hides the options at the top of the menu. As a result, you can't visually compare all your choices; you have to rely on short-term memory. People have enough on their minds, and messing with short-term memory reduces their ability to accomplish tasks on your site. Mega menus show everything at a glance, so users can see rather than try to remember.
More on the subject:Moz.com: 5 Lessons Learned from 100,000 Usability StudiesUserTesting.com: Why You Need to Get Rid of Your Multi-Level Menu
As usual, just my two cents worth of wisdom.