An m3u file, if I remember correctly, is just a text file containing a list (of tracks, URLs, etc.).
You just need to open it with a text editor and manually sort its content into 3 different files.
-- Light a fire for someone and they will be warm for the rest of the day. Set a man on fire and he will be warm for the rest of his life. -Terry Pratchett-
Try to see if MediaInfo (portable version preferred, available here) would provide the technical characteristics of the file, namely the streams or codecs. If it doesn't provide them, it's a bad sign, at least...
Additionally, you need to see if a conversion software (there are many various types) would accept this file, but I'm not sure if that's possible.
Even after searching a bit, I couldn't find any information on an "m3u" being a video file :(
I don't want to be pessimistic, but I think there is little hope :/ However, if it's just for storage, a compression/decompression software (7Zip, WinRAR, WinZip, or others) could split the file, but I don't think that's what you want to do.
Just two last questions: what size (weight) is the file? And are you able to read this file on the computer with a player like VLC for example or another?
With VLC on the PC, there is no problem; I managed to copy and paste a part of the file with VLC, but when I try to copy and paste the other half of the file, surprise, it always saves the first part of the file. It seems that with Notepad, once you have copied one part of the file, you need to reopen Notepad, but personally, I haven't succeeded. Thank you all and have a good evening.