Unique constraint, but only if not null

Is it possible to make an unique constraint that only takes effect when a colum is not NULL?

Here's a example table:

CREATE TABLE tbl(
  col1 TEXT,
  col2 TEXT,
  col3 TEXT,
  UNIQUE(col1, col2, col3)
);

so if col2 is NULL, I want it to be treated like [col1, anything, col3]