Is this possible? I see the ability to select a single value as the default, but can I select say 3/5 values as the default?
From the Multi-Select Picklist Field Type documentation:
Multi-select picklist fields contain a list of one or more items from which a user can choose multiple items. One of the items can be configured as the default item.
So based on the documentation alone, I'd say it isn't possible to select more than one default item.
Idea to promote: Multiple Defaults for Multi-Select Picklist