Hi
This is my first post, I would like to know if you can extend 6/3 wire going from the hot tub to the Spa box, I am short about a foot and I do not really want to have to go buy another length of wire and replace the whole run. If I can extend it I would like to know the proper way of extending it. Thanks for any info anyone can provide.

The code is quite clear on this topic the NEC does not allow the splicing of wiring in between disconnect switch and the hot tub. This wiring is also required to be installed in conduit i.e. non-metallic flexible conduit.
With that being said if you're not selling any time soon and your not getting an inspection, get yourself a waterproof J-box (6"X6" PVC), and 4 large blue wire nuts, and splice away.
Keep in mind that this is not permissible, and will not pass any inspection, and if you are not familiar with how to splice wires ask someone that does to show you.

Doesn't matter. It's not legal in the first place.

"6/3" is cable, and that is NOT allowed AT ALL for ANY outside portion of the wiring to a spa or hot tub.
The wiring MUST be in conduit with a maximum of 6' of flexible conduit.
The main reason for no cable is that the ground MUST be insulated.

If 6/3 is not allowed at all on the outside portion what should I be using? The spa place told me to get 6/3 underground cable which in Canada is quite pricey, was this the wrong cable to get in the first place? If so what should I get in place of it? I would rather have it done some what close to code just in case, also the 6/3 that I am using outside is run through conduit underground and is only out of the ground at the junction box that I put in less than 6 feet from the spa and mounted to the side of the fence off the ground. I guess my biggest concern is if this will work? My GFCI works as intended and the spa seems to be working as intended. Thanks again!

Ah-ha! You are in Canada. That would have been good to know.

Like I said in my previous reply, in the US the wiring MUST be conductors in conduit with an insulated ground.

I don't know the Canadian codes on this so I'll sit back now.