Calculate sum of values of each TextBox in GridView Row and display

I have 1 gridview

In gridview there are 4 asp textbox

textbox 1 = key in number

textbox 2 = key in number

textbox 3 = key in number

textbox 4 = total

how to I sum all the value that are key in  from textbox 1 to 3 and automatically display in textbox 4.

Let say:

textbox 1 = 1

textbox 2 = 2

textbox 3 = 4

total:

textbox 4 = 7

if I key in one of the textbox , the value also appear in textbox 4

textbox 1 = 1

.

.

automatically appear textbox 4 = 1

.

.

textbox 2 = 2

.

.

automatically appear textbox 4 = 3 ...

thanx for helping ...