SELECT jsonb_agg(to_jsonb(idm) || to_jsonb(fk_a) || to_jsonb(p.user_name) || to_jsonb(fk_b) || to_jsonb(k.user_name)) FROM
(select idm, fk_a, p.user_name, fk_b, k.user_name
        FROM public.msg as m
                           INNER JOIN public.tb p ON m.fk_a = p.id_user
                           INNER JOIN public.tb k ON m.fk_b = k.id_user
        WHERE fk_a = 1 AND k.id_user = 3 OR p.id_user = 3 AND fk_b = 1 ORDER BY idm ASC)
	as tbl
;

ERROR:  missing FROM-clause entry for table "p"