As far as I know, there are no chain guides that work with a 3 ring setup, you will need to remove the large ring and set it up as a 2x9 with a bash guard in place of the big ring. There are several options for dual ring chain guides, from different manufacturers. I'm sure you can search pinkbike forums for info on your options.
Not really, why don't you save the money you're spending on a front derailleur and shifter, and use it on a proper chain retention system. It's a Demo 9, it's not meant to have a triple chain ring setup.
It should work in theory (minus chainguide issues) but I'm curious as to why you want to go 3 x 9?
As above, the demo wasnt really designed to run a triple ring set up, what do you want to use it for that you need that gearing? You'd be much better off going 2 x 10 and getting a chain guide, with a wide ratio cassette like a 12 - 36 tooth you'd probably get a pretty similar range of gears as a 3 x 9, only really losing the very highest gears, which probably wont even be used - even top downhillers run a smaller chainring that youd have on a 3 x 9 setup, and they dont seem to run out of gears too often.
Edited to add: I'd recommend either a 1 x 9/10 setup or if you need those gears for your riding, then a different bike, as its probably not what the demo was intended for
Wont the 3rd last big spoke help with climbing? I know its not a climbing bike but just saying...I am mainly gonna be ridin freeride or trail with some drops , I dont like the look of xc or trail bikes I like the DH look on bikes. I currently have the 1x9 set up but just wanna try the specified set-up to see if I like it more. Just wanted to see in this thread if the specs would match up...Worst case senario I just change it back to 1x9