In Psalm 110 …
… David, having written this Psalm, wrote about the Almighty Lord God speaking to the Son of His eternal love,
the Redeemer-Intercessor … the Messiah.
The first Lord is the Almighty Lord God … God the Father.
The second Lord is the “Most Holy Anointed One” who sits at the right hand of the Father.
David, as the writer, was the Earthly King, placed in authority over the people of God, as the Lord of the Earth so to speak. He is the third Lord.
http://www.BibleGateway.com/passage/?search=Psalm+110&version=NKJV
… for more revelation and inspiration … go to these websites.