4 sells setting a ball screen but then uses a screen from 3 to fill the corner. This is as decoy action for 3 to catch at the top of the key.

5 comes to set a ball screen for 3, who attacks the basket or shoots three-pointer. 
3 can also hit 1 curling off the screen that 5 sets immediately after the ball screen.
1 has an open look.
The Celtics used this opportunity to attack the rim via the point guard.