grade1+grade2+grade3/3 is wrong. Division is calculated before addition in C++, so only grade3 is divided by 3 while grade1 and grade2 are not divided.
You have already calculated average, so you should use that instead of the wrong formulas.
Also the condition looks wrong. It seems you should use >= or > (according to the specification) instead of <=.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
