They are all correct but it depends how you use them. Dreams come true you would use like; "I know it might sound weird but, dreams come true" or "You have to make your dreams come true"
Now Dream come true it's all in present sense so it would be used like: "You are a dream come true"
And Dream comes true would be used like: "His dream comes true"
Im sorry I can only try to explain in examples. I hope this helped even just a little.
^^ The above person is correct :)