Next button not working

The Next button can't go anywhere unless you tell it where to go. So you need to keep the "jump to next slide when user clicks Next" trigger. 

The slide advances automatically because that's what the Slide Properties tell it to do: 

The "Slide advances" setting needs to be changed to "By user." 

Other notes: 

You don't need triggers to disable the "Invisible" buttons. You can set the Initial state to Disabled. 

Also, it's possible that a user might click the #3 tab first. Because the #3 layer shows the lightbox that then enables Next, they could advance without seeing layers #1 and #2. So you either need to force them to click the tabs in order, or show the lightboxed video some other way.