I think what your looking for is along the lines of
Code:
If Combobox = Option1 Then
Textbox1.Visible = True
ElseIf Combobox = Option2 Then
Textbox2.Visible = True
ElseIf Combobox = Option3 Then
Textbox3.Visible = True
End If
Replace Combobox with the name of your Combobox and replace Option1,Option2 etc with what is in your combobox.
also replace textbox with the name of the textbox1,textbox2 etc with the name of the text box.
Then place the code in the After Update of the Combobox.