If the third cygnet is the runt, its flight feathers may not be fully developed so it may be a little slower in flying. Give it a couple more days and it should begin to fly with the rest of the family.
Should there be something wrong with the bird and it cannot fly, then it will remain in your pond when the rest of the family leaves for migration during the winter months. At that point, it may be a problem and you may need to see if there is a wildlife sanctuary in the area that might help (and not kill the cygnet if your state is one of the states that insists on killing the Mute Swans to replace them with the Trumpeter Swans for Trophy Waterfowl hunting).
We would give it a few more days or weeks to see if the cygnet begins to fly. More than likely, this is the runt and it will fly soon. The Regal Swan
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