Three decisions that lead to Brutus's downfall include his involvement in the conspiracy to kill Caesar, his refusal to kill Mark Antony, and the fact that he allowed Antony to address the people of Rome at Caesar's funeral. All of these mistakes arise out of Brutus's sense of honor.
Cassius knows Brutus well, and he ruthlessly manipulates him into joining the plot to murder Caesar by appealing to his sense of honor. Cassius himself has no sense of honor: he wants to kill Caesar because he is envious that his former friend and equal is breaking away and becoming the greater man. He doesn't want to have to bow down to him, and so he would rather see him dead. Cassius calculates that having someone with Brutus's untarnished reputation on the conspirators' side will help convince the people of Rome that the assassins' motives are noble and pure. Tragically, Brutus falls for the callous manipulation. He should never have joined the plot.
Once in, the same sense of honor that caused him to join in with bad business becomes his undoing. Cassius knows that the conspirators should murder Antony as well as Caesar, because Antony will be a dire threat to them. However, Brutus refuses this step as unnecessary and overly violent. This is his second main blunder, again based on his innate sense of decency.
Finally, Brutus makes a terrible and almost senseless blunder when he allows Antony to address the mob merely with the guarantee that Antony won't attack the conspirators. Antony does not attack the assassins overtly, but he uses sarcasm and emotional appeals to turn the crowd against Brutus and his cohort, starting a civil war.
Brutus's blunders, ironically, emerge from his genuine goodness as a human being. Because he is an upright and honorable man, he is blind to how low the people around him will sink in their pursuit of their own agendas. He can't see that everyone in the world is not like him.
As an aside, another poor decision that Brutus makes is wearying the soldiers with the long march before the final battle, but it could be argued that by time, the die had long been cast by Brutus's prior decisions.