我的解决方案：

function mergeArraysWithUniqObjectId(...arrays) {
  return arrays
    .flat()
    .filter(
      (el, idx, self) => self.findIndex((inner) => inner.Id === el.Id) === idx
    )
    .sort((a, b) => a.Id.localeCompare(b.Id));
}

let array1 = [
    {
      Id: "car1",
      company: "bmw",
      mileage: 66,
    },
    {
      Id: "car2",
      company: "audi",
      mileage: 53,
    },
    {
      Id: "car3",
      company: "GM",
      mileage: 48,
    },
  ],
  array2 = [
    {
      Id: "car1",
      company: "bmw",
      mileage: 12,
    },
    {
      Id: "car2",
      company: "audi",
      mileage: 67,
    },
    {
      Id: "car4",
      company: "Mercedes",
      mileage: 54,
    },
  ],
  array3 = [
    {
      Id: "car6",
      company: "bmw",
      mileage: 55,
    },
    {
      Id: "car1",
      company: "audi",
      mileage: 67,
    },
    {
      Id: "car5",
      company: "Mercedes",
      mileage: 51,
    },
  ];

console.log(mergeArraysWithUniqObjectId(array1, array2, array3));

像这样的东西可能会工作。
1.复制数组
1.迭代其他数组，添加未找到的项，如果里程数更高则替换。

const res = array1.slice()

array2.forEach(it2=> {
 const index = res.findIndex(it=> it.Id === it2.Id)
 if (index === -1){
   res.push(it2)
 } 
 else if (it2.mileage > res[index].mileage) {
   res.splice(index, 1, it2)
 }
})

const array1 = [{
    Id: 'car1',
    company: 'bmw',
    mileage: 66
  },
  {
    Id: 'car2',
    company: 'audi',
    mileage: 53
  },
  {
    Id: 'car3',
    company: 'GM',
    mileage: 48
  }
]

const array2 =  [
  {
    Id: 'car1',
    company: 'bmw',
    mileage: 12
  },
  {
    Id: 'car2',
    company: 'audi',
    mileage: 67
  },
  {
    Id: 'car4',
    company: 'Mercedes',
    mileage: 54
  }
]

const res = array1.slice()

array2.forEach(it2 => {
  const index = res.findIndex(it => it.Id === it2.Id)
  if (index === -1) {
    res.push(it2)
  } else if (it2.mileage > res[index].mileage) {
    res.splice(index, 1, it2)
  }
})

console.log(res)

我已经使用concat方法连接了2个数组，然后通过for循环找到了唯一的对象。
这里arr1是第一个对象，arr2是第二个对象。

let newArray = [];

let uniqueObject = {};

for (let i in arr) {
  objId = arr[i]['Id'];
  uniqueObject[objId] = arr[i];
}

for (i in uniqueObject) {
  newArray.push(uniqueObject[i]);
}

console.log(newArray);

检查JSFiddle link