According to Unitarians, to whom does the word הָאָדוֹן refer in Mal. 3:1?

I think הָאָדוֹן refers to YHVH in Malachi 3:1. In Hebrew, it says:

הנני שלח מלאכי ופנה־דרך לפני ופתאם יבוא אל־היכלו האדון ׀ אשר־אתם מבקשים ומלאך הברית אשר־אתם חפצים הנה־בא אמר יהוה צבאות

Here is a literal translation, but I'll add punctuation:

"Behold! I send my messanger and he shall prepare the way before me. And shall suddenly come into his temple the Lord whom you seek; and the messanger of the covenant which you delight: behold, he shall come", saith YHVH of hosts.

I see two messangers in this verse. First, the messanger that prepares the way (John the Baptist). The second messanger is "the messanger of the covenant". This isn't John the Baptist because John did not deliver a message about a covenant.

Yeshua says:

"Jesus answered and said unto them, Destroy this temple, and in three days I will raise it up.

Then said the Jews, Forty and six years was this temple in building, and wilt thou rear it up in three days?

But he spake of the temple of his body." John 2:19-21

So the temple YHVH refers to is the body of Yeshua. This gives us:

"Behold! I (YHVH) send my messanger (John) and he (John) shall prepare the way before. And shall suddenly come into his (YHVH's) temple (Yeshua's body) the Lord (YHVH) whom you seek; and the messanger of the covenant (Yeshua) which you delight: behold, he (Yeshua) shall come", saith YHVH of hosts.

I don't know why that Unitarian site said adonai only applies to humans. Here are a few more occurrences that refer to YHVH:

http://biblehub.com/hebrew/haadon_113.htm

As far as this question "After all, doesn't "lord," "my lord," "his lord," "our lord," and "their lord" all refer to the same "lord"?" I would have to say no. Even David called Saul "lord":

"David also arose afterward, and went out of the cave, and cried after Saul, saying, My lord the king. And when Saul looked behind him, David stooped with his face to the earth, and bowed himself." 1 Samuel 24:8

So maybe David was talking about king Saul when he write Psalm 110. But I think it's pretty obvious that David meant a different human lord though.

To assert that "the lord" and "my lord" should refer to the same entity is understandable, but this can be refuted in a logical manner.

Here are a few examples:

My cake is delicious because my mom made the cake with chocolate

In this example, "my cake" and "the cake" probably refer to the same cake

The cake fell on my cake

In this example, we seem to be missing some information; but it's pretty obvious that "the cake" is a separate cake from "my cake" because "my cake" cannot fall on "my cake. Perhaps there was a large vanilla cake made for my birthday party, but my mom made a chocolate cake specifically for me. With this information, the sentence makes perfect sense.

However, these examples involve an inanimate object. A "lord", especially in Psalm 110, is definitely a living being. So here is a closer example:

The boss said to my boss, "Stick with me and you'll be running this whole company one day."

If you heard this, you might be confused for a minute. But once you learn that there is a single owner of the company, and he has a single manager over his store operations, then it becomes clear that "the boss" is the owner and "my boss" is the subservient manager.

If somebody said "Let me get this straight, you have a single boss that exists as three persons, and the first person of your triune boss said to the second person of your triune boss..." Well...I'm not really sure how I would respond to that.

Despite all of this, David makes it easy on us by saying:

YHVH said unto my Lord...

YHVH is the name of the only true God, so there really isn't any kind of issue. This is like if somebody said:

Bob said to my boss...

Conclusion

The author of that article is mistaken. Adonai is used throughout Scripture to refer to both man and YHVH. I have no idea what this has to do with Psalm 110:1, but this is my personal Unitarian interpretation of Malachi 3:1.