Sorry, my Niro has so many "bells and whistles" (literally) that I frequently find myself wondering what it is chirping about. It might help if you described your driving conditions at the time when you hear this sequence of "3 bubble rings". (What is a bubble ring? Sounds like the kind of thing that Tom (RIP) and Ray Magliozzi of CarTalk would have loved to have someone call in and try to vocalize).
For me, the beeping starts when I'm backing out of my garage, because the sidewalls of the garage are so close that the parking assist sensors start chirping away to warn me of objects in close proximity. I also have a feature enabled that occasionally chirps at me to begin coasting to a slower speed, but it only does that when I have a course programmed into the navigation system and it knows that I will be needing to slow down in order to turn at the next intersection (or for some other reason). There are lots of other chirps for highway driving.
If you can't pin it down to a particular kind of driving scenario, maybe you could put your cell phone into recording mode and leave it running until you hear that sound, and then post the result on this forum. I've noticed that most of the chirps I hear have a distinctive tone and meaning, but none of the sounds I can recall fit the "bubble" description (at least, not in my mind).
2018 Niro PHEV Gravity Blue