Document library filter on SharePoint 2010

Hi Everyone,

i am the new for SharePoint.

I have a site collection on that 3 sub sites are there.

                    IT(ss1)            HR(ss2)        Finance(ss3)

Now what i want to achieve is i have a document library named Documentson Site Collection. on that library i have 3 folders like IT,HR and Finance.

I want to use same document library on sub sites (IT,HR and Finance ) also,when ever you are uploading a document on Documents library on main site collection automatically those documents were shown into there 3 sites also. i done that functionality.

Now what i want exactly is when i am going to IT site open that document library, i want to show the onlyIT documents folder only, like this HR and Finance also.

if anyone have idea about this please help me. its very urgent for me.

Regards,

Viswanath.