Answer:
In a quick look there seems to be no real difference. But there is a subtle difference.
A folktale is a story that may have happened and there is evidence to show that various parts of it are true. An example would be Robin Hood. There is evidence that the Locksley's lived at the time of King John and did go on the Crusades but no evidence that he became an outlaw.
A myth is a story that is told mainly to tell a story of natural phenomena. An example of this would be the changing of the seasons. Persephone was abducted by Hades to become his wife and Zeus ordered him to give her back to her mother Demeter. When he said that she had eaten food grown in Hades and therefore couldn't leave under the rules, it was decided that she would reside in Hades for 6 months and spend 6 months with her mother. This explained why there was a summer (Persephone with Demeter) and a winter (Persephone in Hades).
A legend is a story usually about a person(s) or object which cannot be proved to have existed in any way, shape or form. An example of this would be King Arthur of the Britons. The first accounts of him were documented over 600 years after he was supposed to have lived, no evidence can be found of where Camelot was situated (even though many places say that it was situated near them), no physical evidence such as a grave (he was taken to the Isle of Avalon by boat to die or rest as the legend later goes on) and various archeological digs have come up with no evidence for any of the stories associated with him. The legend later goes on that King Arthur isn't dead but resting and will return when Britain is in danger. The last bit of this legend that makes it a legend is that Arthur is King of the Britons. Even in Medieval times, the people of Britain weren't called Britons until 1707 when the Act of Union joined both England and Scotland under one King/Queen and one Parliament.