I was trying to add it the same way I had added the other two MP3s; i.e. by clicking on Add Background Audio". In the ensuing pop-up, I selected Add to Music Track... that's where I encountered the error message. I'm pretty sure I was positioned correctly in the timeline and I had more than enough video remaining (about 4+ minutes of video for an MP3 that was a little over 3-mins long).
I got around the problem, somewhat inelegantly, by deleting the two MP3s already loaded, clicking Add Background Audio, and selecting all 3 MP3s at once. Lost a bit of work as I had done some front and back end editting on the first two MP3s but it was much better for my mental health to go back and re-edit them than to keep trying to figure out why I simply couldn't add the third one on its own.
Thanks for jumping in to try and help though; I do appreciate it. Have a great night,