In short:A trio has to pass the ball to each other in a square.That is tricky yes, because one is always missing....
Organisation:Three players stand in an imaginary square about eight metres from each other. One corner remains unoccupied. The middle player (number 2) of the three has the ball.Start in square. In from (basic) Later: 1 under the post.
Basis: always offer sides. Not in a clump, not too early, not too late.