I would have thought this one was reasonably simple to do unless I am misunderstanding something. To not show categories 1,2 & 3 just remove them from the chart. To include a sum of category 1,2 & 3 add another category to the chart which is formed from an expression which is Fields!Category1.Value + Fields!Category2.Value + Fields!Category3.Value
This is very simple so I apologise if I have misunderstood something and told you what you already know. If this is the case, provide a bit more information on the problem and I'll see what I can do to help.