3-6-3 indicates the order the ball was touched by the players to the end of the play.
Pitcher (1 / P)
Catcher (2 / C)
First baseman (3 / 1B)
Second baseman (4 / 2B)
Third baseman (5 / 3B)
Shortstop (6 / SS)
Left fielder (7 / LF)
Center fielder (8 / CF)
Right fielder (9 / RF)
Thanks for the explanation!