in the entity file

/**
 * @ORM\Table(name="unique_test",uniqueConstraints={@ORM\UniqueConstraint(name="unique_idx", columns={"col1", "col2"})})
 */
class UniqueTest
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $col1;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $col2;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $col3;