The ungodly, is the person who lacks God in their life. They can typically be the unbeliever or even be referred to as a believer who lacks spiritual resources.
The sinner, generally refers to the believer who lacks spiritual resources. They just go about their business as though they have all that they need, and care nothing for God, truth, or anything for that matter, but too, they can be the person who presumes that they know it all, or have enough already in the spiritual realm, and are in need of nothing further.
The scoffer, is the person who makes fun, either by indifference and disregard for doctrine, or through, out and out rejection of doctrine.
In any case, all three references belong to this verse as people who have no weight, no substance, nothing of value in their life.
In the agriculture world, the grain is the valuable portion of the crop and the chaff is the worthless, weightless portion, that just a little breeze can blow away.
When winnowing a crop, the harvest is dropped or tossed into the air, and against the wind, and the grain drops down, while the chaff is blown away or drifts away with the air.
And so it is in life. God drops us all sooner or later, and those who have value will fall into His hands. Those who lack value, will drift away into uselessness.
Unlike the well cared for tree, which is cared for by another, with well placed roots, and access and desire for the water of life, the chaff has no roots, has no desire for true nourishment, no weight value, and therefore produces no fruit or production of any type. The wind, even a slight breeze can blow the chaff away due to its lack of substance.
The unbeliever is the worst of the chaff, because he has no value whatsoever, has no desire for truth, and has no means of gaining truth.
The believer who disregards truth, thinks that he does not need truth, believes that he needs no instruction, fails to grow up as well, and likewise will stand before the Lord in a worthless state.
Refer back to verse one. The only person who acquires happiness’s, is the one who wants to learn, places themselves in a learning environment, listens, learns, and grows up in their spiritual life. By utilizing the doctrine that they learn, they develop wisdom and spiritual production, and therefore stand ready before Christ, for phenomenal blessings for all of eternity.