Obtener el valor entre la 2.ª y la 3.ª coma

select 
  regexp_substr('2901 MAIN ST,CORNING,NY,14830', '(.*?,){2}(.*?),', 1, 1, '', 2) 
from dual

En general,

n_th_component := 
  regexp_substr(string, '(.*?,){'||(n-1)||'}([^,]*)', 1, 1, '', 2);

Ejemplo:

select 
  n,  
  regexp_substr('2901 MAIN ST,CORNING,NY,14830', 
                '(.*?,){'||(n-1)||'}([^,]*)', 1, 1, '', 2)
from (select level n from dual connect by level <= 4)