"The third time's a charm!" isn't the best line to use when you're toasting the newlyweds. The couple would probably prefer to keep their romantic histories under wraps in front of hundreds of people, and there's no better way to kill the buzz of a magical evening than to talk about relationships that didn't last.
The point of the toast is to focus on the success of the couple and to congratulate them on their new union. Concentrate on talking about the qualities they have that will make their marriage last!