Indexes and sorting

Are indexes useful for speeding up ORDER BY clauses? Example:

CREATE TABLE t
(
a INT,
b INT,
c INT,
d INT
);

SELECT * FROM t WHERE a = 1 AND b = 2 AND c = 3 ORDER BY b;

Let's say the table just has one index:

CREATE INDEX b_idx ON t (b);

In this case, obviously the b_idx will be used and no sorting after the fact
will be required. Now let's add an index:

CREATE INDEX key_idx ON t (a, b, c);

On the same query, now the key_idx will be used and there'll be a sort
wrapped around it all. The question is, is the b_idx useful at all anymore?
Can it be used to speed up the sort step? If so, how? If not, why not?

-John