What is the difference in usage between 势必， 必定 and 必然?
势必 be bound to; certainly will 


势必 = 形势必然 (situation will certainly be) 

势必(compound word):bound to

Example: 决赛:德国对巴西, 到時势必有一场恶斗
(Final match: Germany vs. Brazil, when the time comes, there will certainly be a tough fight) 


emphasize on:'the situation dictate a certain outcome' (the situation of Germany vs. Brazil ensure a tough fight) 



  必定 to be bound to; to be sure to


必= certain; 定= 肯定 = sure 

必定 (compound word):certainly; surely

Example: 每次德国遇上巴西, 都必定有一场恶斗
(Whenever Germany runs into Brazil,  they would certainly have a tough fight) 


emphasize on 'an absolute certainty' (The fact that 'Germany vs. Brazil is always a tough fight' is an absolute certainty)



  必然 inevitable; certain; necessity


必= certainly; 然= naturally

必然 (compound word):certainly

Example: 如果德国遇上巴西,那必然是一场恶斗
(If Germany ran into Brazil, then it would certainly be a tough fight) 


emphasize on 'a natural outcome of certainty' (the nature outcome of Germany vs. Brazil is a tough fight)