I would say that there will be some from each group. There will definitely be Christians and Jews there considering that there are canonized Christians and Old Testament Jews. As for Muslims, I am sure there will be some there who followed what their conscience told them about God to the best of their ability while being invincibly ignorant of Christianity. It is probably the same with some Jews as well.
Thats according to you but not according to Bible It clearly says several times that you should follow the prophet to come
New Testament
Gospel of John chapter 16 verse 12-14:
"I have yet many things to say unto you, but ye cannot bear them now. Howbeit when he, the Spirit of truth is come, he will guide you unto all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. He shall glorify me".
Gospel of John chapter 16 verse 7:
"Nevertheless I tell you the truth; it is expedient for you that I go away: for if I go not away, the Comforter will not
come unto you; but if I depart, I will send him unto you".
Old Testament
God speaks to Moses in Book of Deuteronomy chapter 18 verse 18:
"I will raise them up a Prophet from among their brethren, like unto thee, and I will put my words in his mouth; and he shall speak unto them all that I shall command him."
However, it is Prophet Muhammad (pbuh) who is like Moses (pbuh):
i) Both had a father and a mother, while Jesus (pbuh) was born miraculously without any male intervention.
[Mathew 1:18 and Luke 1:35 and also Al-Qur'an 3:42-47]
ii) Both were married and had children. Jesus (pbuh) according to the Bible did not marry nor had children.
iii) Both died natural deaths. Jesus (pbuh) has been raised up alive. (4:157-158)
Muhammad (pbuh) is from among the brethren of Moses (pbuh). Arabs are brethren of Jews. Abraham (pbuh) had two sons: Ishmail and Isaac (pbut). The Arabs are the descendants of Ishmail (pbuh) and the Jews are the descendants of Isaac (pbuh).
Isaiah chapter 29 verse 12:
"And the book is delivered to him that is not learned, saying, Read this, I pray thee: and he saith, I am not learned."
When Archangel Gabrail commanded Muhammad (pbuh) by saying Iqra - "Read", he replied, "I am not learned".