Is there an Iran-N. Korea-China nexus? A web search about China's connection with North Korea's relationship with Iran suggests that there are many (several thousand) published reports on the subject. If the reader chooses not to believe these reports (including reports based on Wikileaks docs), that is his call. There is no way to verify any claim regarding top secrets of governments. However, historically the Korean peninsula was China's most significant satellite and since the Korean War China has supported N. Korea for its own geopolitical interests. I am not in a position to provide top secret documents as proof for what I write, but this is what we know from published reports:
a) Iran has a long-standing relationship with N. Korea, at least this is what US claims and there seems to be many reports on this matter. The UN sent a report on N. Korean missile transfer to Security Council in November 2010. US had been pressing China to block such shipments, instead of facilitating them; a story that has a history predating Obama.
b) Russia and China at the very least monitor Iran's relationship with N. Korea, and at most facilitate and support.
c) US has directly confronted Russia over the alleged missiles transfer issue, primarily because the N. Korean missiles are modeled after Russian missile technology. US has been pressing China not to permit the ballistic missile transfer.
d) While it is true that American officials over-estimate Beijing's influence with N. Korea, without China there is not much leverage that N. Korea has in the world. China is the most important ally of N. Korea. N. Korea depends on China not only for diplomatic and strategic support, but for trade, food, fuel and of course weapons. Without China, N. Korea under the current regime simply cannot survive.
The US has accused Chinese firms in May last year of supplying Iran with a key chemical weapons precursor and assistance with operating a chemical manufacturing plant. It may very well be the case that Iran does not have a military relationship with N. Korea as the US, UN, and other governments and private organizations claim. It may all be a massive propaganda campaign for all we know. And it may be the case that Russia and China have no clue of whether N. Korea has any type of a military relationship with Iran (or Syria). China, Russia and of course Iran categorically deny any transfer of missiles, let alone missiles going through Chinese soil. In my view, it stands to reason that China and Russia use their influence with both N. Korea and Iran to counterbalance US strategic influence in the Middle East and of course in East Asia. But unless Beijing and Moscow actually admit their actual roles in N. Korea and Iran, it is impossible to say with certainty.
"The US also accused Chinese firms in May last year of supplying Iran
with a key chemical weapons precursor and assistance with operating a
chemical manufacturing plant."
"China, Iran and North Korea have established a strategic alliance that focuses on missile and nuclear development, according to a new report."
"Secretary of State Hillary Clinton in May, expressing concern that exports by named Chinese firms "could be used for or diverted to a CW program." Clinton asked whether the suspect transfers were approved by the Chinese government and warns that sanctions may be imposed. "We request that the Chinese government take all steps necessary to investigate this matter and to prevent Iran from acquiring
dual-use equipment and technology that could be used in its CW program."
The cable noted that the United States had raised its concerns with Chinese officials on numerous occasions and listed at least 10 instances in which it said North Korean shipments of ballistic missiles parts to Iran passed unimpeded through Beijing.
"A CIA report covering 2004 indicates that Iran continued to receive “ballistic missile-related cooperation” from entities in North Korea as well as Russia and China. However, foreign assistance enabled Tehran to “move toward its goal of becoming self-sufficient in the production of ballistic missiles,” the report adds. Safavi claimed that Iran no longer requires foreign assistance for its missile programs." http://www.armscontrol.org/act/2007_01-02/IranNK