when you say "define in bulk" do you mean define multiple item validations at once? In this case the answer is no.
If you refer to validations on form level instead of item level, then the answer is yes. Form Validations run after all item validations and can be of type PL/SQL where you can check whatever you need.
I think "multiple not null validations" were available in apex 3.2 , don't remember seeing it in apex 4.0.
<li>If you are on Apex 4.0, you don't need to create not null validations separately for items, you can set that at the item level itself.
Go to Item Properties -> Settings section -> Value Required. This would automatically cause a validation to be performed on that item.
Else you can create a page validation and use an SQL expression like
:ITEM2 IS NOT NULL
OR :ITEM2 IS NOT NULL
OR :ITEM3 IS NOT NULL