Design a queue that supports `push` and `pop` operations in the front, middle, and back.

Implement the `FrontMiddleBack` class:

- `FrontMiddleBack()` Initializes the queue.
- `void pushFront(int val)` Adds `val` to the front of the queue.
- `void pushMiddle(int val)` Adds `val` to the middle of the queue.
- `void pushBack(int val)` Adds `val` to the back of the queue.
- `int popFront()` Removes the front element of the queue and returns it. If the queue is empty, return `-1`.
- `int popMiddle()` Removes the middle element of the queue and returns it. If the queue is empty, return `-1`.
- `int popBack()` Removes the back element of the queue and returns it. If the queue is empty, return `-1`.

Notice that when there are two middle position choices, the operation is performed on the frontmost middle position choice. For example:

- Pushing `6` into the middle of `[1, 2, 3, 4, 5]` results in `[1, 2, 6, 3, 4, 5]`.
- Popping the middle from `[1, 2, 3, 4, 5, 6]` returns `3` and results in `[1, 2, 4, 5, 6]`.

Example 1:

    Input:
    ["FrontMiddleBackQueue", "pushFront", "pushBack", "pushMiddle", "pushMiddle", "popFront", "popMiddle", "popMiddle", "popBack", "popFront"]
    [[], [1], [2], [3], [4], [], [], [], [], []]
    Output:
    [null, null, null, null, null, 1, 3, 4, 2, -1]

    Explanation:
    FrontMiddleBackQueue q = new FrontMiddleBackQueue();
    q.pushFront(1);   // [1]
    q.pushBack(2);    // [1, 2]
    q.pushMiddle(3);  // [1, 3, 2]
    q.pushMiddle(4);  // [1, 4, 3, 2]
    q.popFront();     // return 1 -> [4, 3, 2]
    q.popMiddle();    // return 3 -> [4, 2]
    q.popMiddle();    // return 4 -> [2]
    q.popBack();      // return 2 -> []
    q.popFront();     // return -1 -> [] (The queue is empty)

Constraints:

- `1 <= val <= 109`
- At most `1000` calls will be made to `pushFront`, `pushMiddle`, `pushBack`, `popFront`, `popMiddle`, and `popBack`.