Of the 3 employees, the main person is the shop manager。The remaining two are the booking clerk and the cashier.
「 3人の内1人は店長、残りの2人予約係とレジ係です。」は英語で「Of the 3 employees, 1 is the shop manager, and the remaining 2 are the reservation clerk and the cashier.」という表現が良いと思います。
店長ーShop manager
予約係ーBooking clerk・Reservation clerk
レジ係ーCashier
アメリカ合衆国 
