。
A three-mile race is scheduled for tomorrow.
Literal
Tomorrow 3-mile race [subject-が] be-held scheduled [polite-です].
行われる is the passive form of 行う ('to carry out/conduct'), used to mean 'be held' for events — the passive removes the agent, emphasizing the event itself. マイル is an English loanword — miles are rarely used in Japan (km dominates), so this is a sports-specific or English-influenced context.