That's the tradeoff - borderless scaled printing without margins or actual size printing with margins. Sounds like your problem really likes with paper selection and fitting. 10" rolls would be ideal and might warrant a paper change. Perhaps another orientation would better optimize placement? 2 wide in landscape orientation on a 24" roll for example...
Otherwise getting 3 wide on a 24" will require some tricky math where you size the images a little smaller and then print with borderless to get the actual size. That's a complicated road to go down and makes for an ugly workflow...
Also, if you're doing a decent volume, you might consider a nesting solution like that from ImageNest, Qimage or even the Free Layout that comes with your 8300. Photoshop is a slow dog for volume work.