I'm not sure what you are trying to do, so I suspect E.G. will have as much trouble. You have data which is granular at a given level, but you want to display it at a less granular level. So it seems the first thing you need to do is tell SAS how it is to assess the source data you have. Use a Data Summary task to create a new table with three hour granularity and populate it with your 15 minute data. To do that you'll need to decide whether your 3 hour slots will show SUM, MEAN, MAX, MEDIAN or some other value, and you will then have a table you can graph.
Use the Pie Chart task to take your data and plot it. I'm not clear on why you'd use a pie chart for this purpose, it seems to me that it will be difficult to read and interpret. A pie chart with slices sounds to me like a display of a clock face, but when the slice size will change with the data value it will be a very strange sort of clock. In your situation, I would use a bar chart which would be easily interpreted by any reader to distinguish differences between each 3 hour slot.