Go to debug mode >> Decimal Accuracy >> Product Prices = 3 Digits

Go to Sales Order Form >> Studio >> Edit the Unit Price field name to something like "Unit Price 1" so that Odoo creates the XML in the view, "Odoo Studio: sale.order.form customization". Edit Quantity field as well. 

Edit the XML lines to include the <attribute name="digits"> [12,2] </attribute> as shown below:

The digits [12,2] represent 12 digits before decimal and 2 digits after. This will work on fields of type float.  

The system will use 3 decimal places, but will do a round-off on the Sales Order view due to the field being constrained by the XML.

 
A similar change can be done on PDF reports to change the decimal precision on the report via studio. 

1. Select the float field that needs a change in precision.

2. On the properties pane, select 'float' as the widget. 
3. A new property will show up - 'decimal precision' -- enter the precision that you require for the field on the report