I print a lot of graphics and at that size I would expect the file to be 24x36. You could create the graphic at 25% scale but there isn't really a good reason to do that. But you will start to run into problems when your artboard gets around 200 inches. If you want a 24x36 graphic, then yes, I would create the artboard at that size. I say this so your print vendor doesn't mistakenly print the graphic at the incorrect size.
I would export the graphic as a PDF unless you have a specific purpose for needing a jpg file. Illustrations in Illustrator should be saved as a PDF so you can retain vector quality, which would allow you to scale the graphic to any size. If you have a image inside your illustration and upscale the image, then you will lose quality.