How to batch write to Firestore on a single document with 2 or more custom class objects

I have 3 classes:

K1{int field A, int field B} 
K2{bool fA, bool fB, bool fC} 
K3{String fA}

And I want to write them into a single Document in a Firestore. When I try:

K1 k1 = new K1...
K2 k2 = new K2...
K3 k3 = new K3...
WriteBatch writeBatch = db.batch();
writeBatch.set(documentReference, k1, SetOptions.merge());
writeBatch.set(documentReference, k2, SetOptions.merge());
writeBatch.set(documentReference, k3, SetOptions.merge());
writeBatch.commit();

only 1 of them gets written. If those were 3 different documentReference things would work, but not if it's same one. Of course, I could try simply:

documentReference.set(k1, SetOptions.merge()).addOnSuccesListener(
   ...
   documentReference.set(k2, SetOptions.merge()).addOnSuccesListener(
      ...
      documentReference.set(k3, SetOptions.merge());
   )
)

and that would have worked, but it would count as 3 writes to DB.

Is there a way to do a batch write to the same document with a combination of multiple custom objects, or even a HashMap and a custom object?

You cannot use a batch to update a single document with multiple values, as the document will only contain only the most recent data that was written.

If you need to have a document that looks like this:

docId (document)
 |
 --- k1 (object)
 |   |
 |   --- a: 1
 |   |
 |   --- b: 2
 |
 --- k2 (object)
 |   |
 |   --- b: 3
 |   |
 |   --- c: 4
 |
 --- k3 (object)
     |
     --- d: 5
     |
     --- e: 6

All you have to do is to create another class:

class Doc {
    public K1 k1;
    public K2 k2;
    public K3 k3;

    public Doc(K1 k1, K2 k2, K3 k3) {
        this.k1 = k1;
        this.k2 = k2;
        this.k3 = k3;
    }
}

And then create a new instance of the class like this:

Doc doc = new Doc(new K1(1, 2), new K2(3, 4), new K3(5, 6));
documentReference.set(doc);

Even if you pass three objects to the constructor, you'll always have to pay a single write operation.

On the other hand, if you need to update multiple fields within the same document, then you should pass those values to the DocumentReference#update() method, as explained in the official documentation:

• https://firebase.google.com/docs/firestore/manage-data/add-data#update_fields_in_nested_objects

P.S. Remember that a batch write is useful only when you want to perform operations at multiple different locations.