If the argument here is about the word carnal one has to determined what exactly is meant by it in the context it is being used. Certainly in the Corinthian church there were those who was exhibiting traits of the lost (carnal) in certain of their actions.
The passage in Romans 8 is talking about a carnal mind, not simply carnal actions. The carnal mind is one that has no desire to follow God and the things of God are foolishness to them. In the case of the Corinthian's in chapter 3 Paul is dealing with certain actions in certain believers that are carnal in nature and he names what they are, and if they are really saved they would deal with them once confronted or pointed out as such. One problem is however even though a person could be a Christian and exibit the things mentioned in chapter 3 it does not mean they are. It all depends on if they come out of these things once confronted. In fact in this case they must not have come out as we read in 2 Corinthians where Paul questions their salvation.
Also in chapter 6 Paul warns that there are certain actions where a person who claims to be saved those actions makes his/her claim false and those things are listed. He even goes so far as to warn them/us not to be deceived and think that this claim of carnality and salvation can be extended to every area of life or sin. It cannot.
So was there some carnal actions on the part of some in Corinth? Yes, but also true is that if they were engaged in anything mentioned in the list in chapter 6 they were not really Christians.
These are possible actions of a believer who is carnal;
For ye are yet carnal: for whereas [there is] among you envying, and strife, and divisions, are ye not carnal, and walk as men?
These are those who are lost;
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,
Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God.