A website run by exiles in Thailand said that Shwe Mann's visit was to procure arms, discuss tunnel-building projects and other matters.
Among the pictures purportedly leaked by the three men were those of an alleged secret network of tunnels built by North Korean experts inside Myanmar, which were published in June by the Democratic Voice of Burma (DVB), based in Oslo, Norway.
There has been speculation that the tunnels are part of secret nuclear facilities being developed with the assistance of Pyongyang.
During a visit to Thailand last July, Hillary Clinton, the US secretary of state, said North Korea could be sharing atomic technology with Myanmar.
The documents further showed that Maung Aye, the government number two, visited Russia in 2006 to discuss the procurement of a guided missile system, the DVB said on its website on Friday.
The military government, which took power in 1962, has not commented on the verdicts or the accusations surrounding the leaked documents.
The death penalty is rarely carried out in Myanmar, with sentences normally commuted to life imprisonment.