物体在3s内的位移为：x3＝v0t3+

1

2

at32＝5×3+

1

2

×0.5×9m=17.25m．
物体在2s内的位移为：x2＝v0t2+

1

2

at22＝5×2+

1

2

×0.5×4m=11m．
则物体在第3s内的位移为：△x=x₃-x₂=6.25m．
故答案为：17.25m，6.25m．