Jupyter means "Julia, Python, R", the three original languages that it was created for. Nowadays you can add support for many languages by using other kernels.
It wasn’t ´created’ for the three languages at all.
Originally the whole project was ‘IPython’ and then ‘IPython Notebook’. It came out of Fernando Perez’s attempt to make interactive Python a better experience.
They renamed it Jupyter to reflect that it was no longer a pure Python project as kernels were written in even before it rebranded as Jupyter. Those three were the most popular at the time but by no means the only ones.
I used to work with one of the core team about the time they changed this!
If you want to see the available kernels for each language you can check it here: https://github.com/jupyter/jupyter/wiki/Jupyter-kernels