assign child a form to third forms

I have been developing a windows application. I have a problem to assign child to the main form.
In my project
Main Form(Mdi Parent)
Second Form(Child of Main Form)
Third Form(Open from Second Form)
When i opened the third form from second form i want to assign the third form as a chid to Main Form.I couldnot assign. still now i am having this problem.Can any one suggest plz?