Is Malachi 3:1 calling Jesus Christ a Messenger or a Angel of the covenant?
First of all, Malachi 3:1 is about John the Baptist, not about Jesus. Like John said about himself, “I am the voice of one calling in the wilderness, ‘Make straight the way for the Lord.'” (John 1:23) In Matthew 11:9-10 Jesus said that John the Baptist is the fulfillment of the prophecy, “I will send my messenger ahead of you, who will prepare your way before you.” By the way, Malachi 4:5-6 is also a prophecy about the coming of John the Baptist to prepare the way for Jesus. The very last words in our Old Testament is an announcement about the coming of John the Baptist, who will introduce Jesus to the world.
So, you can see that this passage, although related to the coming of Jesus, is about John the Baptist preparing the way for Jesus. As for the word used there, the Hebrew word for angel can also be translated as messenger. Therefore, whether this word is translated as angel or messenger is determined by the context. The context of Malachi 3:1 is a prophecy about a human who is to come, not an angel, so, naturally, the translators use messenger, not angel.
I have a feeling that you are interacting with someone who believes that Jesus was a powerful angel. The Jehovah Witness group believe this. If so, then their proof text is off target for two reasons in this case–because it is not about Jesus, and it is not about an angel.