I've got a GL3 that I sideloaded RelaunchX, ALreader, Colordict, and Solid Explorer. Everything works really well for me. I did accidentally set RelaunchX to always open when I hit the home button and it didn't brick my device.
You don't need to root to side load apps just ADB but I rooted mine so I could remap the hardware keys. More info on ADB and rooting can be found here:
https://forum.xda-developers.com/noo...ght-3-t3802331.
I'll try and find some time to install Perfect Viewer to see if it works.