counting between dynamic textboxes

In my userform I have dynamic text boxes. Each row consists of three text boxes. In each row I want to have a formula(for example: TextBox1 + TextBox2 = textbox 3) and a change-event if a change a value. The problem is that I don't now where and how to give that change-event, because the textboxes are dynamically added (and the number of textboxes is i). Hope that I made myself clear and somebody can help me

mabulco

Here's the code.
with sdlc I count the number of columns in use. The number of rows with boxes is j (1 to sdlc - 1 [ first column not relevant]).

What i want is some sort of update where firstcol j + secondcol j = thirdcol j.

Sub boxes(formname)

On Error Resume Next

Dim ws As Worksheet
Dim j As Integer

Set ws = ThisWorkbook.Sheets("sheet1")

sdlc = ws.Cells(1, Columns.Count).End(xlToLeft).Column

For j = 1 To (sdlc - 1)

    With formname.MultiPage1.page3.Controls.Add("Forms.textbox.1", "firstcol" & j, True)
    .Top = (25 * j) + 20
    .Left = 12

    End With

Next j

For j = 1 To (sdlc - 1)

    With formname.MultiPage1.page3.Controls.Add("Forms.textbox.1", "secondcol" & j, True)
    .Top = (25 * j) + 20
    .Left = formname.Controls("firstcol" & j).Width + 24
    End With

Next j

For j = 1 To (sdlc - 1)

    With formname.MultiPage1.page3.Controls.Add("Forms.textbox.1", "thirdcol" & j, True)
    .Top = (25 * j) + 20
    .Left = formname.Controls("firstcol" & j).Width + formname.Controls("secondcol" & j).Width + 36
    End With

Next j

End Sub