由q=It可得:I= Q t = 3C 10s =0.3A; 由Q=UIt可得:U= Q It = 18J 0.3A×10s =6V;由欧姆定律可得:R= U I = 6V 0.3A =20Ω;当该导体两端电压为10伏时,因导体的电阻不变,∴I′= U′ R = 10V 20Ω =0.5A,∴5秒内通过某个导体横截面的电荷量为Q′=I′t′=0.5A×5s=2.5c.故答案为:0.3;6;2.5.