We love it when "old" devices get updated to new versions of Android, especially when said old devices are really great smartphones that were popular in their day and still serve their job even in today's landscape. My LG G3, for example, is now in my mother's hands and she has zero complaints about it since it does more than she could ever need. I'm still waiting for Marshmallow to hit her phone though, because as much as she doesn't care what Android version runs below the surface, I do want her to have Doze to improve the aging battery's life and to get a newer Android Security Patch Level (which is sure to come with a full system update).
My mom has to be patient (or I do, actually), but the wait is getting shorter for you or your parents if your G3 is on T-Mobile or AT&T. We've received tips from users on both operators telling us that they got Android 6.0 Marshmallow on their phones. On T-Mobile, it seems that the update isn't rolling out OTA just yet, but is available if you connect your phone to the LG PC Suite. Both our tipster and this T-Mo user support thread corroborate that. On AT&T, we're not so sure. You can check for an OTA update or you can try connecting to the PC Suite to see if it's available there.
Both T-Mobile and AT&T's support documents for the LG G3 still state Android 5.0.1 as the latest software version so it's possible these are limited soak tests available to select users before the update is ready to be pushed to everyone OTA.