Personally this is my view. I think this was popular in the middle ages (it’s mentioned in Dante’s Divine Comedy), but as far as I know it’s never been church doctrine.
Those that are Christians and saved will go to what is called Heaven, with God. Those who deny and hate Christ, oppose him without remorse, and sin against God and his creation, will be in hell. Their punishment will be equal to their crimes. A lot of Islamic terrorists and atheists will end up in hell.
The middle group are those that were neither believers nor opposers. I think there is a sort of limbo or intermediate state for them. They are not punished but not rewarded with the Kingdom of God, either. They are on their own. These are those people who had the opportunity to become Christians but simply chose not to, but they are not hateful towards Christ. Again their particular circumstances depend on their conduct in accordance with God’s law in life. I don’t think these people go to “heaven” per se, because to me an essential element of faith is admitting one’s sinfulness and one’s inability to atone for them on one’s own. When I was religion shopping no other faith offered redemption in that manner.
I don’t think those who never had the chance to hear the Gospel will suffer. I think they go to a place of perfect natural happiness.