You call them the thirdbaseman
The umpire is the person that rules on plays.
You would call a person who plays the French horn a "hornist."
A "harmonicanist" is what a person who plays the harmonica is called.
PianistA pianist is a person who plays the piano.
A person who plays in movies is a actor or an actress
Such a person is a Violinist.
A person who plays Scrabble is called a Scrabbler. (unofficially)
I guess you can call that person an athlete.
A drummer or percussionist plays the drums.