How did Magna Carta, the Hundred Years' War, and the Black Death change European society?

All three of the events in your question contributed to the downfall of the feudal social system of Europe.  Magna Carta was written to protect the rights of citizens from tyrannical  monarchs.  It checked the power of the king and gave more influence on government to the nobles.  This weakened the position of kings and emboldened the noble class.  The 100 Year's War introduced different technologies to warfare that undermined the importance of knights in the feudal system.  The 100 Year's War also further weakened the power of the king as people started to look for parliaments to have more influence on government, especially in England.  The Bubonic Plague had a devastating effect on all classes in the feudal system.  Because death did not discriminate, many powerful manor lords died and their estates were increasingly left behind.  Additionally, the plague had depleted the population of serfs and created a labor shortage.  This gave the lower classes more bargaining power and better wages.  All three of these events greatly diminished the system of feudalism and led to more representative forms of government.