Your formal statement of the argument is one possible way to present it.
1)There is an omniscient, omnipotent, omnibenevolent God.
(2)If there is a 3 O God, then he would know about evil(or suffering) (omniscience), have the power to stop it (omnipotence), and want to stop it (omnibenevolence).
(3)Therefore, there would be no evil(suffering)
(4)However, there is evil(suffering) in the world
(C)Therefore, there is no omniscient, omnipotent, omnibenevolent God.
However, I would question your conclusion that the only way it can be refuted is to attack #4. Also, I would question your equating evil with suffering. Suffering may actually be a good since it may lead to betterment of individuals. Equating the two makes your conclusion quite false.
The problem of evil only effects the Christian idea of a 3 O God, and has no bearing on a deist creator. The only way to refute the argument is to attack claim (4), which some have attempted to do.
It is possible to reformulate your argument in this way to arrive at quite a different conclusion
(1) There is an omniscient, omnipotent, omnibenevolent God.
(2) A 3 O God is not bounded by time or space.
(3) If a 3 O God exists, he exists outside of time and space.
(4) Evil (human suffering) exists only as a temporary condition within time and space.
(5) If there is a 3 O God, then he knows about evil (or suffering) because of his omniscience, has the power to stop it because of his omnipotence, and wills to stop it because of his omnibenevolence.
(6) Since evil exists within time and space, its existence can be ended in time and space.
(7) A 3 O God could allow temporary evil (suffering) if its existence in time and space also allowed the 3 O God to bring about a greater good outside of time and space.
(C) Therefore, a 3 O God could allow temporary evil (suffering) to exist.