I have made considerable progress !
It was like this: At first I made one desperate attempt. I took a new SD card and wrote the LE image for the Orange Pi One Plus on it.
I inserted the card into the Orange Pi 3 and turned on the power. LE and Kodi started normally. I made some settings and installed (among other things) the PVR IPTV Simple Client add-on and my IPTV add-on. I used it for a few days, waiting for a total freeze to occur again. It didn't happen!
So it looks like the freeze problem is probably related to either USB3 or WiFi / BT. To rule out at least one of these, I decided to run at least USB3 (the absence of USB3 bothered me much more than the absence of WiFi and BT). So I found the source file sun50i-h6-orangepi-one-plus.dts and tried to compile it into sun50i-h6-orangepi-one-plus.dtb. But I couldn't. I was still getting the error message "FATAL ERROR: Unable to parse input tree".
So I did it differently. I took the sun50i-h6-orangepi-one-plus.dtb file located in the /flash folder and converted it to the sun50i-h6-orangepi-one-plus.dts file. There I enabled USB3 and did the conversion back to sun50i-h6-orangepi-one-plus.dtb.
I replaced the original file in the /flash folder and turned on the device. It worked - USB3 started working.
I used the device with the modified DT file again for a few days - it never froze!
Then I took the next step. I returned to the original SD card I had earlier in Orange Pi 3. There I replaced the original sun50i-h6-orangepi-3.dtb file with the modified sun50i-h6-orangepi-one-plus.dtb file. After switching on the device, the expected is confirmed: no total freezing.
Result: It seems that the total freezing of the Orange Pi 3 is related to either WiFi or BT. However, it is not enough if I use the original file sun50i-h6-orangepi-3.dtb and disable WiFi and BT in the settings of the LibreELEC add-on.
How do I proceed to find the real cause of the freeze?
Note: I'm still talking about "total freezing". So I say a state where the board did not respond to anything (except turning off the power or pressing the power button). This never happened while testing the device with a modified DT file. However, sometimes (always during IPTV playback) there was a temporary freeze (the image stopped and after a while Kodi restarted spontaneously). This is probably related to the mentioned ffmpeg. Interestingly, this crash never occurred on an Orange Pi 3 board with the original sun50i-h6-orangepi-3.dtb file.