Please check the output file name is wrong you have outpuy.Thats is why you are getting error.Could you tell me where you post questions.
Run the following command to get the file from hdfs:
$ hadoop fs -get /user/cloudera/class3/wc-output/part-r-00000
this command will copy the file from hdfs into your local directory in Linux filesystem.
From your command, it seems like you are trying to download the file from hdfs to local directory.
As @dinesh suggested, you have to use the -get command.
$ hadoop fs -get /user/cloudera/class3/wc-output/part-r-00000 /home/cloudera/class3/
Hope this helps.