dynamically adding textbox

Hi dear friends, I have a question,

 

i want to add two or more than two textboxes dynamically at run time and save its value to database. I want to add textboxes in following position. the first 3 textbox appears by default and after clicking add button more three textbox below the first 3 textbox should be added

 

textbox1     textbox2    textbox3      ADD BUTTON

 

textbox4     textbox5    textbox6      ADD BUTTON

 

textbox7     textbox8    textbox9      ADD BUTTON .......... 

 

thank you,  would appreciate help