What are the Big 3 of the non-mainstream churches?

Answer 1

The Seventh Day Adventists with 16 million members.

The Church of Jesus Christ of Latter Day Saints with 13.8 million members.

Jehovahs Witnesses with 7.3 million members.

All three are distinct from what is regarded as "mainstream" Christianity, while still being regarded as "Christian" by many.

All three were founded in the United States and achieved their initial growth there. All three were originally dismissed as "cults", and some still accuse each of them of that. All three have since branched out to become international movements.

The Seventh Day Adventists are perhaps the most similar to ordinary Christian doctrine, with some notable exceptions. They worship on Saturday, the "Sabbath" or "Seventh day", hence their name. They espouse vegetarianism and did so a century in advance of any other group's interest in such. They had a female prophet, Ellen White, who wrote a book called "Prophecy in America", which while not a replacement Bible, does supplement it.

The Church of Jesus Christ of Latter Day Saints (LDS or Mormons) departs further from ordinary Christian doctrine in that they believe that their prophet, Joseph Smith, was visited by an angel who revealed not only new doctrine, but a "new" New Testament to the Bible called "The Book of Mormon". They believe that Jesus visited the Americas after his ascension. They at one time believed in plural marriages, and some off shoots of them still do. They are against the drinking of Alcoholic Beverages and smoking and drinking stimulants such as coffee or tea.

Jehovahs Witnesses are perhaps the furthest away from mainstream Christianity while still believing in the Bible and that Jesus Christ was the son of God. They believe it is very important that God be called by his name "Jehovah", and that heaven is only for 144,000 "elect" people, the rest of the saved getting to work to build a "Paradise Earth". They are fairly insular, in that they do not marry outside their faith, and typically only associate with those in their faith. They are best known for their extensive missionary efforts, principally knocking on people's doors to offer bible studies or copies of their Watchtower magazine.