You could also call a change event that is a little simplier.

Sub Multiply_Value()
On Error Resume Next
textbox4.value = textbox1.value * textbox2.value * textbox3.value
end sub

and then on the change event for each of the 3 textboxs, call the Multiply_Value command.

The only difference in this one, is if any of the 3 boxes do not contain a value, then box4 will not generate a value.