Number of Decimal Places for Purchase Order Quantity

I need to setup 3 digits decimal place for fields like purchase quantity and other related fields (receive, invoice, remaining quantity etc). i have read some related posts in the forum , but still not clear about setting decimal place to more than 3. I changed the baseline decimal place to 3. But when i type 3 decimal place in PO Quantity it is changing to 2 place.  Please suggest an answer