Three merchant ships loaded with Chinese-made arms, including Silkworm missiles, are believed to be en route to Iran, Reagan Administration sources said in Washington. U.S. intelligence shows two ships left North Korea--one more than a week ago and the other only a few days ago, the sources told United Press International. A third sailed from Shanghai in recent days, heading southward toward the Indian Ocean, they said. Analysts are reportedly convinced that the two ships from North Korea are headed toward Iran but think it is possible that the third may be delivering arms to Iraq. China has consistently denied U.S. charges that it has supplied arms to Iran, helping to perpetuate the seven-year Iran-Iraq War.