It seems that Professor Third is a single parent, as his daughters' mother never appears in the series. The exact reason why is unknown and there's been no mention or explanation to confirm officially whether he's divorced, separated or widowed. He's first seen towards the end of Red Robins Don't Fly, when the show's main safety patrollers are at the Thirds' house playing cards and he serves them hamburgers.
His only other appearance is in  In this, he comes down with a cold and his daughters are trying out various remedies to cure him.
He was mentioned in  by Ingrid that he's working on a book about a 17th century Japanese mathematician and that she'll spend her spring break helping to translate it into the language of the same name.
- "A Cold Day at X."
- "South of Friendship, North of Honor"