The CM 13 Nightly builds is now live for all the LG G3 Sprint models. If you want to install this custom ROM on your device, then you should check the guidelines listed in this post.
DownloadsCM13 Nightly for LG G3 Sprint - Download
Gapps for CM13 - Download
How To InstallStep One: Copy downloaded files to your phone's sdcard root (/sdcard)
Step Two: Reboot to recovery mode, wipe data and cache
Step Three: Flash rom, then flash Gapps
Step Four: Reboot device to system