* China launches Shenzhou-12 in first crewed mission since 2016

* Three more crewed missions to complete China's space station

* China aims to finish building space station by end-2022 (Adds astronauts boarded module)

By Carlos Garcia and Shubing Wang

JIUQUAN, China, June 17 (Reuters) - Three Chinese astronauts on Thursday flew to an unfinished space station in China's first crewed mission since 2016, expanding the country's already growing near-Earth presence and challenging U.S. leadership in orbital space.

The astronauts rode to Tianhe - the module that will be the living quarters of China's completed space station - on Shenzhou-12, or "Divine Vessel". The crew boarded the module, where they will live for three months, the longest stay in low-Earth orbit by any Chinese national.

China's space station, due to be finished by end-2022, will be the only alternative to the two-decade-old, U.S.-led International Space Station (ISS), which may be retired in 2024.

If the ISS - backed mainly by the United States, Russia, Japan, Europe and Canada - is decommissioned, China would be the operator of the only active space station. That would potentially give it greater power in shaping future norms and regulations for near-Earth space, which is already teeming with Chinese satellites.

"At this current stage, we haven't considered the participation of international astronauts, but their future participation will be guaranteed," said Zhou Jianping, chief designer of China's manned space programme.

"I'm aware that many countries have expressed their wish in this regard," Zhou told foreign reporters at the Shenzhou launch site in northwestern Gansu province.

Shenzhou-12 is the third of 11 missions - four of which will be crewed - needed to complete China's first full-fledged space station. Construction began in April with the launch of Tianhe, a cylinder-like module slightly bigger than a city bus.

The Shenzhou-12 astronauts Nie Haisheng, 56, Liu Boming, 54, and Tang Hongbo, 45, will test out technologies on Tianhe including its life-support system. They will also be monitored for how they fare in space physically and psychologically. An upcoming mission to the space station will last six months.

Barred by U.S. law from working with NASA and by extension on the ISS, China has spent the past decade developing technologies to build its own space station, in addition to planning missions to the moon, Mars and other planets.

China plans to allow Hong Kong astronauts to join future missions, Zhou also said.