Three people called in sick this morning.

called 是动词 call 的过去式，意思是"打电话"。 in 在这里是介词，表示"处于某种状态"。 sick 是形容词，意思是"生病的"。 called in sick 是一个固定短语，意思是"打电话请病假"，指员工因生病而打电话通知雇主无法上班。