You are welcomed to check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1247.0]EJS version of Vector addition (sum of two vectors)[/url],too!
This java applets try to show : How to add two vector A and Binto vector C
C = A + B = B + A
How to play:

1. There are two methods to form a vector
2.    1. Vector A/B originate from center of the window

Left click anywhere inside the window to form vector A or B
2. Vector A/B originate from where you first  click the button

Drag and lift the left mouse button to form vector A or B     Vector will move to center of the window.

• The program will show you how to add two vector A and B into C

•

 C = A + B or C = B+ A C = A - B or C= -B + A

A - B can be transformed to A + (-B)

• press reset button or right click the mouse button to reset.
•
Parameters are reset to default values.

• select info checkbox to show more information.
•
X,Y components of each vector ...
How to process more than two vectors :

 SUM = A + B + C + D = (A + B) + (C+D) = E+ F E = A + B and F= C + D
Could you figure out how to process SUM = A + B + C ?
3-D version

The following is a flash demo for the above java applet.

How to form a 3_D vector
Move the mouse with the window will establish a vector at X-Y plane.  Left click the mouse, then drags it up/down to change the Z components. Then release the mouse button

When you form two vector A and B
Another vector C = A + B will show up.
Drag the right mouse button to change the viewing angle. Drag near the origin to change the its position.(displacement)