I'm not entirely sure on this one, but I believe flow charts are a visual representation of the data you are using for your application, and how it is inputted and / or manipulated. In this case,
you receive the input record (either three floats for each exam mark, or they can be in a structure....try searching a previous post for more info on structures).
from there, you must manipulate the data...so you add the numbers together. finally, the results are outputted.
I believe the idea of flow charts is to make you comfortable thinking about a program as real-life 'objects' in terms of how they interact. In real life, from this description you have three main tasks: get the data, produce the results, and display the results (give the student the mark or something similar).
Translating this into a program, you can write it the conventional way (everything inside your main function, which works for smaller applications), or group the related tasks into functions. Lookup functions for an idea of how they are used.
Try to see if you can start writing some of this yourself and don't get discouraged. Do as much reading as possible to familiarize yourself with all the definitions/terms of the language as you can grasp right now.
Good luck! hope this information helped you about a bit (i apologize if there is any ambiguity in my explanation)....I believe there's a post a few pages back with a VERY similar program to the one you are asked to write
Grade program