auto summarizing consecutive groups of rows

[Mikap]

I have entered numerical data in a consecutive row of cells:

A1 12
B1 10
C1 8
D1 13
E1 12
F1 11
G1 10

What I would like to do is be able to select consecutive ranges by using
some sort of fixed sized (hopefully using highlighted or contrasting color)
block or frame.

For example:

Highlight: =sum(A1+B1+ C1);
Press "tab" and get the block or frame to move down a full cell at a time;
Highlight: =sum(B1+C1+D1);
Press "tab" and get the block or frame to move down a full cell at a time;
Highlight: =sum(C1+D1+E1);
etc.

To make matters even more complicated, I would also like to be able to
change the values in previously selected groups of cells so they will change
to "0". In other words, when I reach the "block: =sum(D1+E1+F1), I would
like the data in each of the cells A1, B1 and C1 to change to "0".

I am very new to Excel so I don't know if any of the above functions are
even possible. Feedback will be greatly appreciated. Thanks