In a certain code language, 'world largest snake' is coded

Solution

I: 'world largest snake' is coded as '138'.
II: 'largest uncovered fossils' is coded as '534'.
III: 'rivalled world fossils' is coded as '914'.
From statement I and III, we conclude that 'world' is coded as '1'.
From statement I and II, we conclude that 'largest' is coded as '3'.
From statement I alone, we conclude that 'snake' is coded as '8'.
From statement II and III, we conclude that 'fossils' is coded as '4'.
From statement II alone, we conclude that 'uncovered' is coded as '5'.
From statement III alone, we conclude that 'rivalled' is coded as '9'.
So, the code for 'rivalled snake uncovered' is '985'.