Pls help me !

a value got as an output in (textbox3) by using two text box values(textbox1 and textbox2) .but i want an answer in (textbox4)by using that value(textbox3) and one text box value(either textbox1 or textbox2)

Facing a difficulty that when i got the output in textbox3 itself i need that value for another calculation is there any event or solution to use that value . Actually i tired the text box changed event for this but its not working