Textbox update from other form

[jdy0803]

Here is 3 forms, form3, form1, form2.
form1 has textbox.
A. Start form3, form3 load form1, form1 load form2 and form2 try updating form1.textbox but failed.
B. But start form1, form1 load form2 and form2 try updating form1.textbox and then textbox is normally updated.

I don't know why B succeed but A fail.
How do I do to succeed in A?
(I attached project to show this.)