One of the easiest ways to adjust brightness on an Android device is to use the slider found in the notification shade. Unfortunately, for some odd reason, the Sprint variant of the LG G3 does not have this feature.
Searching extensively through the Settings, I found nothing that could enable this on my device. There is a Quick Settings option with brightness presets, but this only allowed for 0, 50, and 100% brightness, along with one custom preset of my choosing.
Thankfully, I found an alternative to adjusting brightness, and in my opinion, it's even better than having a traditional slider. By enabling one specific setting in G3 TweaksBox, you can slide across the status bar to adjust your G3's brightness without any fuss.
Before starting, make sure you have a rooted LG G3 with Xposed Framework installed. Then, make sure to install the free G3 TweaksBox by developer P_Toti from the Play Store and activate it with the Xposed Installer app afterward. G3 TweaksBox, as the name implies, actually has a lot of cool tweaks you can perform, but I'll just be focusing on brightness.
With G3 TweaksBox open, head into Theme Tweaks -> Status Bar. At the top of the page, hit Enable Status Bar mod, then slide down to the bottom of the page and check Enable Status Bar sliding.
With the tweak enabled, just slide across your status bar to brighten or dim your G3's display. It's not the conventional slider, but it might actually be better as it saves space in the notification area. Plus, you don't have to open any menus or settings. Just slide the status bar to dim or brighten your screen. That's it. Pretty awesome, right?
As mentioned before, G3 TweaksBox is no one-trick pony. You can tint your status bar, tweak status bar icons, theme you nav bar, tweak your notification tray, and more. Feel free to explore and let us know which tweak you find most useful.