How to use the same trigger function for insert/update/delete triggers avoiding the problem with new and old objects

I am looking for an elegant solution to this situation:

• I have created a trigger function that updates the table supply with the sum of some detail rows, whenever a row is inserted or updated on warehouse_supplies.
• PostgreSQL insert or update syntax allowed me to share the same function sync_supply_stock() for the insert and update conditions.
• However, when I try to wire the after delete condition to the function it cannot be reused (although it is logically valid), for the returning object must be old instead of new.

-- The function I want to use for the 3 conditions (insert, update, delete)
create or replace function sync_supply_stock ()
returns trigger
as $$
begin
    -- update the supply whose stock just changed in warehouse_supply with
    -- the sum its stocks on all the warehouses.
    update supply 
    set stock = (select sum(stock) from warehouse_supplies where supply_id = new.supply_id)
    where supply_id = new.supply_id;
    
    return new;
end;
$$ language plpgsql;

-- The (probably) unnecessary copy of the previous function, this time returning old.
create or replace function sync_supply_stock2 ()
returns trigger
as $$
begin
    -- update the supply whose stock just changed in warehouse_supply with
    -- the sum its stocks on all the warehouses.
    update supply 
    set stock = (select sum(stock) from warehouse_supplies where supply_id = old.supply_id)
    where supply_id = old.supply_id;
    
    return old;
end;
$$ language plpgsql;

-- The after insert/update trigger
create trigger on_warehouse_supplies__after_upsert after insert or update
on warehouse_supplies for each row 
execute procedure sync_supply_stock ();

-- The after delete trigger
create trigger on_warehouse_supplies__after_delete after delete
on warehouse_supplies for each row 
execute procedure sync_supply_stock2 ();

Am I missing something or is there any fixing to duplicating sync_supply_stock2() as sync_supply_stock2()?

EDIT

For the benefit of future readers, following @bergi answer and discusion, this is a possible factorized solution

create or replace function sync_supply_stock ()
returns trigger
as $$
declare
    _supply_id int;
begin
    -- read the supply_id column from `new` on insert/update conditions and from `old` on delete conditions
    _supply_id = coalesce(new.supply_id, old.supply_id);
    
    -- update the supply whose stock just changed in of_warehouse_supply with
    -- the sum its stocks on all the warehouses.
    update of_supply 
    set stock = (select sum(stock) from of_warehouse_supplies where supply_id = _supply_id)
    where supply_id = _supply_id;
            
    -- returns `new` on insert/update conditions and `old` on delete conditions 
    return coalesce(new, old);
end;
$$ language plpgsql;

create trigger on_warehouse_supplies__after_upsert after insert or update
on of_warehouse_supplies for each row 
execute procedure sync_supply_stock ();

create trigger on_warehouse_supplies__after_delete after delete
on of_warehouse_supplies for each row 
execute procedure sync_supply_stock ();