Blockchain - my first look

 I have one pirate ship P and three army ships A B C.
 A B C must agree on common time to attack P. In order
 P to be destroyed it has to be at least 1 : 1 battle.
 
 So we have a traitor in army ships B which receives
 the message from A that the attack time is 21:00.
 Instead of sending that to C, B fabricates attack time 
 to 23:00 and sends it to C. 
 
 Now as A go to attack mode, B joins pirates P and wins
 the battle because its 2 : 1. After that the battle is
 2 : 1 against C. So B hacked the system. Preventions?
 
 
 How C can detect that info was fabricated? 
 The only info source for him is B.