In Oracle Forms the hierarchy looks like this:
block -> item
So in this case the whole block except FIELD_THREE can be updated:
SET_BLOCK_PROPERTY ('MY_BLOCK', UPDATE_ALLOWED, PROPERTY_TRUE);
and
SET_ITEM_PROPERTY ('MY_BLOCK.FIELD_THREE', UPDATE_ALLOWED, PROPERTY_FALSE);
In this case the whole block cannot be updated:
SET_BLOCK_PROPERTY ('MY_BLOCK', UPDATE_ALLOWED, PROPERTY_FALSE);
and
SET_ITEM_PROPERTY ('MY_BLOCK.FIELD_THREE', UPDATE_ALLOWED, PROPERTY_TRUE);
This is why an user is unable to enter a value in the FIELD_THREE.