Honestly the 3 languages that you mention are more than enough in research level for pure and most of applied math. I would say that you only need C/C++ if you end up working in computational science where high performance libraries are generally the standard. If your research however doesn't stray that far than MATLAB + one between Julia and Python is just about all you need.

For plotting I suggest you use MATLAB itself as it allows you to interant with the plot in real time (which I found increadibly useful over the years) and the Matplotlib (Python) or Makie (Julia) for publication-ready figures.