text changed event

I have 4 text boxes. My requirement is whenever there is a text changed event happens for the textbox 1 2 or 3. I need to put the changed value in textbox4 in such a way that. The order should be 1 and 2 and 3. How should I achieve this? I am using vb.net web app
Textbox4.text = textbox1.text and textbox2.text and textbox3.text