Change Hotbar Items in Gamemode 3

How do i do this?, I tried setting the items to the players hotbar (0,1,2,3) but they don't replace the items that are there for Gamemode 3, instead when you open your inventory they're there, did a little bit of google searching and couldn't find it.

If it doesn't work I'll find out how to do noclip.

Edit: Hold up, I could of made a dumb testing
Edit 2: Nah Didn't work

Solved Prefix removed, Thought i had the answer but I didnt

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

bump

 

I don't think you can with Inventory#setItem(). If I'm right, it's a client side thing, so maybe there's a packet for it (Look for it in the spigot files), but you can do what most of the servers do, instead of spectator mode, you could use adventure mode with invisibility and fly ability. It solves the going into walls problem too.