Application Express :: Dynamically Update A Number Field By Summing Of Page Item Values

I'm trying to update a number field on an apex page by summing up the values of multiple page items. I've tried to follow this tutorial

[URL]........

especially the "Create a Set Value Dynamic Action Using PL/SQL" part.

In the tutorial example they return an page item value P3_SAL multiplied by a multiplier they've determined by another page item's case. What I want to do is much simpler in that I just want to add up multiple page item values and then display them in a number field at the bottom of the page.

so the code I tried to use based on the tutorial is in the tab: Home >Application Builder>Application 103>Page 3>Edit Dynamic Action>Create / Edit Action

Set Type: PL/SQL Function Body

PL/SQL Function Body:

BEGIN
return :P3_ITEM1+P3_ITEM2+P3_ITEM3;
END;

Page Items to Submit: P3_ITEM1,P3_ITEM2,P3_ITEM3

But it gives me an error that I need to declare the identifiers, yet it works if I only use one item and I can perform any arithmetic.

i.e.

BEGIN
return :P3_ITEM1+1000;
END;