Meaning:- The belief that the third time something is attempted is more likely to succeed than the previous two attempts. It is also used as a good luck charm - spoken just before trying something for the third time. Origin:- The first citation I can find for it is in The Weekly Sentinel, June 1912. This is in a rather snooty court report about a Mrs. Martha Carliss, who had been twice married previously: That Mrs. Martha Carliss evidently believes in peace and happiness in wedlock and that she probably thinks third time's a charm is shown by the fact that she was granted a license today to marry Andrew W. Mowery. Precursor to this phrase is in Elizabeth Barrett Browning's Letters addressed to R. H. Horne, 1839: 'The luck of the third adventure' is proverbial.
One more proverbial is "The third time's lucky" listed in Alexander Hislop's The proverb of Scotland. Why is third time lucky? There are few suggestions.
The most common is that it alludes to the belief that, under English law, anyone who survived three attempts at hanging would be set free. This is probably from the story of John 'Babbacombe' Lee. Another suggestion is that it refers to the Christian Trinity. There seems little to support that idea. It might relate in some way to goodness or luck being associated with the number three, but has no specific link with the third of anything.