I think that one's already enough.

The description mentions that the three values will be distinct.
Initial Solution Typo fixed.

Thanks for the comment, Tuhaj. Updated, though I believe that this is not the place to introduce new concepts. To the new programmers it should just remind that there is such thing as threads, and if they want to solve this (very-very basic) kata they should read about it. Writing a test case themselves is as important part of the learning process as solving the problem. I always add sample tests for more complex cases, but in such simple ones I often leave it to the reader. Anyways, I may be wrong, so I added a sample test case, an example to the description and two pointers - to Ruby Threads module and to the wikipedia page about threads.