Absolutely not! The angel of the Lord in the Old Testament is the pre-incarnate Jesus Christ and he first appears as the angel of the Lord at Genesis 16:7 to Hagar. Please read the rest of the chapter from vs 7 to vs16 and notice what she says at vs13.
His primary purpose in the Old Testament was to be a mediator to Israel, guide them and protect them. Hebrews 1:1-2, "God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, vs2, in these last days has spoken to us in His Son, whom He appointed heir of all things, through whom also He made the world."
I want you to notice something that many people miss from Hebrews 1:1. What's the very first word? "God!" God spoke to the fathers and prophets inspiring them what to say.
Now, regarding Acts 10:3 it says, "About the ninth hour of the day he clearly say in a vision (an) angel of God who had just come in to him and said to him, "Cornelius."
Most Bibles use the word "a" angel or "an" angel and once in a while "the" angel is used. There is a big difference between the words, "a/an" and the word "the."
The chief grammatical function of "an" (or a) is to connote a thing not previously noted or recognized, while "the" connotes a thing previously noted or recognized.
Going back to the Old Testament "the" angel of the Lord is mentioned on numerous occasions as "the" angel of the Lord and is identified at times as God, does things that only God can do and speaks in the first person as God.
At Acts 7 where Stephen is reading the "riot" act to the Jews he says at vs30, "And after forty years had passed, "An" angel appeared to him/Moses in the wilderness of Mount Sinai, in the flame of a burning throne bush." Yes, Stephen did use the word "an."
Now look at vs38, "This is the one who was in the congregation in the wilderness together with "the" angel who was speaking to him on Mount Sinai, and who was with our fathers; and he received living oracles to pass on to you." These "oracles" are "sayings" and is generally used of revelations of God, or here the Ten Commandments are called "living" because they provide guidance for the conduct of life and because they had their source and authorship in the living God.