Strange behaviour of SELECT ... IN

when I do a:

explain select * from table1 where name in ('JORGE');

NOTICE:  QUERY PLAN:

Index Scan using idx_table1 on table  (cost=0.00..10901.87 rows=3184 width=48)

but if do a:

explain select * from table1 where name in ('JORGE',  'JUAN',  'JOSE');

NOTICE:  QUERY PLAN:

Seq Scan on table1  (cost=0.00..16689.73 rows=9506 width=48)


why??? is there any way to make postgresql use index in the second type of
query?

thnx!

Jorge S.