Aggregating CSV Data

[nb1214]

I have a CSV file that I will be working with, and it contains a lot of data, and two of the columns are "Company Name" and "Sales". I would like to add a "Sales Group" column, which divides all of the companies on the sheet in thirds based on their "Sales". The top 1/3 of companies based on sales would have "High Sales" in the "Sales Group" column, while the middle 1/3 and lower 1/3 would have "Medium Sales" and "Low Sales", respectively. I would like to keep the rest of the data the same, and just add this new column.

I have attached a sample CSV file so it is more clear what I am after. Thank you in advance for any assistance.
  Data Python (version 1).csv (Size: 292 bytes / Downloads: 301)