None of the three men you named ever sat before McCarthy's committee
That is false. Roy Cohn, the brilliant young lawyer who was Sen. McCarthy's chief assistant, asked Cedric Belfrage whether he had been a communist at the time he was performing his postwar duties, or if he was was at present. Belfrage declined to answer, citing his Fifth Amendment privilege.
Funny also is the fact that investigators within the past couple of years have found that Cedric Belfrage was actually a double agent - while passing innocuous documents to the Soviets, he was gaining info for MI6 back in London.
You cite no authority for that claim, and I doubt it. The Venona decrypts show the information he provided the USSR was anything but "innocuous."
Solomon Adler supplied info to the Soviets about the Maoists, not about American projects or plans.
The FBI had good reason to think otherwise. That is why Adler is one of the persons the bureau discussed in a fifty-page report entitled "Soviet Espionage in the United States, November 27, 1945." Adler was identified as a communist by Elizabeth Bentley and Whittaker Chambers, both of whom were former communists with invaluable, accurate inside information about the Soviet espionage network in the U.S. He was the sidekick of an even more notorious Soviet agent, John Stewart Service. Both of them tried through a steady stream of reports to Washington to influence U.S. policy, fervently advocating Mao Tse-tung while reviling Chiang Kai-Shek. During the mid-1940's, Service and Adler had been housemates in Chungking with Chi Chao-ting, another Soviet agent. Adler was a second-tier figure in the "Silvermaster Circle" of Soviet agents the FBI knew was operating in the Treasury Dept. and studied in great detail in its reports.
Here are two more communist agents who testified before Senate committees McCarthy was involved with: V. Frank Coe, whose Soviet cover name Venona reveals was "Peak," and Harold Glasser, who had the fitting cover name "Ruble." They also were members of the circle of agents operating in the Treasury Dept.
The names of these two came up in connection with plates to print occupation currency in postwar Germany, redeemable by the U.S. Elizabeth Bentley had testified that her former Soviet bosses had ordered these plates to be transferred to Soviet control, and that soon after, communist agents in the Treasury Dept. had gotten that done. Coe had written a memo passing along a Soviet request for more dies to print money, and when he was asked if he was engaged in espionage activities for the Soviet Union at the time he wrote it, he took the Fifth and declined to answer. He did the same when asked if he was currently a communist.
Glasser did much the same. When one of McCarthy's staffers asked him if he was a member of the Communist Party at the time he attending meetings which concerned the currency plates, he took the Fifth and declined to answer. And when asked if, at the time he attended those meetings, he was engaged in espionage, Glasser also declined to answer on the ground that it might tend to incriminate him.
I am not denying that there were Soviet agents operating in the US from the late 30s up until the collapse of the Soviet Union but none of the really effective ones were ever brought before McCarthy
Even if that were true, which it is not, what difference would it make? None. In his speeches, Sen. McCarthy identified many important figures in the Soviet espionage effort in the U.S. in addition to the ones the committees on which he served heard testimony from. He led the effort to make the American people aware of a very serious penetration of various agencies of the U.S. government by a vast network of communists and communist sympathizers working for Stalin's Soviet Union and the Red Chinese against our security interests.
But for Joe McCarthy's brave, patriotic efforts, for which a cast of cowardly mutts arranged his destruction to cover their own lies and dereliction of duty, this enormous breach of our national security would have gone largely unnoticed. The fact their comrades were unmasked dismays leftists today, so they continue to parrot claims about Joe McCarthy that have been proven false by mountains of new information. It's hard to be sure if they do this out of ignorance, dishonesty, or both. But they think nothing of peddling falsehoods as part of their frantic effort to cover the backsides of their fellow leftists, just as the Alinskyite Mrs. Clinton thinks nothing of piling lie upon lie in a frantic effort to cover her own ample rear.