> Say I wanted to present a list of users with their top three to-do items, that's impossible using the above method.

Not really:

    SELECT u.user, i.text
    FROM users u,
    LATERAL (SELECT * FROM items WHERE items.user = u.user ORDER BY items.pos DESC LIMIT 3) i;

(You can also make these columns instead of rows, but as someone already pointed out, you'd usually not do that in SQL.)