Don't use .TEXT You don't need to use that in 99% of the places. Use the .Value property instead as it does NOT require the control to have the focus. So, you would use:
Code:
Private Sub Combo98_AfterUpdate()
Me!OQT = Me!Combo98.Column(1)
Me!JCO = Me!Combo98.Column(2)
Me!CN = Me!Combo98.Column(3)
End Sub
As far as the read only part - you need to make sure that the record source for your form is updatable. What are you using for your form's record source? Is it a query? If so, what is the SQL for it?