。
The baseball club has three managers.
Literal
Baseball-club [gen-の] manager [topic-は] 3 [counter-人] [copula-です].
野球部 — 部 is the standard suffix for school clubs: 野球部 ('baseball club'), 美術部 ('art club'), 吹奏楽部 ('brass band'). マネージャー in a Japanese school sports context often refers to a team manager/assistant (sometimes a female student in a male club), a role distinct from the English business 'manager.' The counter 人 (にん) counts people.