Many inventory organizations under the same Ledger

Hi -

assume that i have the following structure

- One Business Group

- One Legal Entity

- One Ledger

- Three Operation Units

- Three Inventory organisations

-The three inventory organizations are process enabled .

- there are a lot of items in common between the INV organizations .

- We are using PMAC.

1- What if the same item has different costs in each organization ? which one would be posted in GL  ?

2- period end closing process in OPM has to be done for each one of them Separately ?

3- If i used [Cost organizations] and shared the cost for common items ..what would be done for items which are not in common ?