vbs字符串的取值判断，怎么用VBS脚本编写

public static int GetStr(string str)
{
    string s;
    int lastX = str.lengh + 1;
    int lastY = str.lengh;
    int lastZ = str.lengh - 1;
    char a = str[lastZ];
    char b = str[lastY];
    char c = str[lastX];
    s = a + b + c;
    return;
}
// 获得字符串abcdefg 中的  efg
GetStr("abcdefg");