The bottom photo of the young tree gives us a better clue. If you look up Golden Chain Tree in the Database, known as Laburnum or Laburnum anagyroides, you will see a photo by Skiekitty that shows the 'leaves of three' in a more angular form. If you scroll to the bottom of the tree you will see the green bark.
These trees, with their long hanging chains of yellow blossoms, can throw many seedlings quite easily, from the pendulous flower and seed casings. Enjoy their beauty in the late spring.