As you can see, we have three sets of three here. Nine. I immediately think of nine choirs of angels, ordered into three sets of three, as pointed out by the Angelic Doctor, the Common Doctor, the great Saint Thomas Aquinas. See Question 108. The angelic degrees of hierarchies and orders. A good read, even for this most un-well-read priest.
There is no greater rejoicing of the angels in heaven than over one sinner who converts. It strikes me that they express this joy by, for instance, giving flowers of sinners like yours truly to the Immaculate Conception, Queen of Angels.
We can rejoice in their rejoicing. It’s important to rejoice. Even and especially in stormy times. Always. If we can have, by grace, some bit of purity of heart and agility of soul, then we are immediately brought to this rejoicing, for we see just how far the Divine Son of the Immaculate Conception had to reach into this world to get us, which is cause for rejoicing. He had to stretch His arms in both directions on the cross, under which stood His dearest mother and ours. The angels are agape. And then they rejoice.