My Saturday was busy and I ended up folding laundry on Sunday. Is this a violation of the Third Commandment?

As a general rule, there is a precept that we refrain from engaging in work or activities that hinder the worship owed to God, The joy proper to the Lord's day, the performance of the works of mercy, and the appropriate relaxation of mind and body. (Catechism #2185). 
 
That said, we also do well to avoid an excessive legalism against which Jesus himself taught when he said The Sabbath was made for man, not man for the Sabbath. (Mk 2:27) 
 
Hence, while one might ordinarily seek to avoid folding laundry etc. on a Sunday, such activity is not intrinsically wrong. Perhaps one finds such an activity relaxing in the company of other family members. Perhaps too, since the activity could not reasonably be accomplished on Saturday, it is an act of charity that helps the family to be prepared for the week ahead. 
 
Thus, we do well to seek a proper balance between maintaining the principles of joyful rest on Sunday, and avoiding excessive legalism.