Psa 136:1 Oh, give thanks to the LORD, for He is good! For His mercy endures forever.
The LORD is the "Tetragrammaton" YHWH which is the "memorial" name of God, THE NAME. It means literally, "I who am, was, and always will be". Perhaps a good English translation is The Eternal One. Most scholars feel that the pronunciation of this name should be YAHWEH
Psa 136:2 Oh, give thanks to the God of gods! For His mercy endures forever.
Here both words are Elohim and mean literally "mighty ones". When Elohim is accompanied by the singular article or used with a singular verb it becomes more abstract and not plural. So, in this case "God" literally means the "Almighty". This becomes more understandable when we realize that "elohim" in the plural is applied in scripture to angels and human judges. The Elohim is supreme above all other elohims no matter how powerful they think they might be.
Psa 136:3 Oh, give thanks to the Lord of lords! For His mercy endures forever:
"Lord" and "Lords" here is the Hebrew word Adon and means a sovereign. God is the supreme Sovereign of the universe, far superior to all others whether human or angelic.
YAHWEH is depicted as the supreme power in the universe and yet with all of this power He remains both good and merciful. For this, He is to be praised and worshipped.
I hope you have found this helpful.