Obtain different random() values with generate_series() in PostgreSQL

I am trying to generate random data (in volume) and was trying to use PostgreSQL's random() function with generate_series(), as shown:

SELECT
  (SELECT random()) AS a,
  (SELECT random()) AS b,
FROM generate_series(1,3);

However, **I get the same random values per row**:

a   |   b   
-------+-------
 0.124 | 0.443 
 0.124 | 0.443 
 0.124 | 0.443

How can this be solved?