I'm not expert enough at Android yet to give full information, but here is my experience, which might be relevant also to later phones than the G3: I'm using a D855, with fulmics Marshmallow, rooted. The fulmics Tweaks allow the call recording on-screen button to be enabled (default disabled). On making a call or on one coming in, the record call screen button is enabled. It works fine for me. Calls are saved in /storage/emulatd/0/VoiceRecorder/my_sounds with filenames like "CALL_+44 20 7974 4444_0.3gp"; they play back fine. I transfer them to a PC, where they can be played with VLC.
As there is an LG G3 voice recorder application, I'd expect somehow to be able to enable it for calls, possibly requiring rooting, with a stock ROM, but that's only a guess..
Regarding legality: in the UK it's not illegal to record your incoming and outgoing calls without notification; it's illegal to provide the recording to a third party if caller was not notified. This is not standardised across the EU countries.
The recording is in .gp3 format (which I hadn't heard of). It can be opened on PC, Linux, and Mac by VLC. It seems to be a high-quality format (higher than required for phone calls): VLC says codec MPEG AAC audio (mp4a), stereo, sample rate 44100Hz; a 1'9" call made a 1.1GB file. If I need to keep a lot of recordings I'll move them from phone to my general backup drive, and maybe compress them with handbrakeCLI.exe. HTH