finding words that contain only 3 characters using sed in a file

I need to print only words that consist of 3 characters, however the word document is a numbered list. Here's the exact question that I have to answer: > Using the sed command with [[:lower:]] character class on the animals file, find all the animal names that are only three characters long _(3 marks)_. This is what i have tried: cat animals | sed '/{[:lower:]].../d' cat animals | sed '/{[:lower:]]/d' sed '/[[:lower:]]{3}/d' animals This is the file I am trying to find the words from (the animals file): 01. aardvark 02. badger 03. cow 04. dog 05. elephant 06. fox 07. goose 08. horse 09. iguana 10. jackal 11. koala 12. lamb 13. mongoose 14. narwhal 15. onyx 16. pig 17. quail 18. rat 19. snake 20. tiger 21. umbrellabird 22. vulture 23. walrus 24. xerus 25. yak 26. zebra i have just found out the code cannot have the [[:lower:]] in it more than once is there a way to do this??