Didnt FO3 have a chinese faction? The opening shows the chinese landing in America with a huge paradrop, how come their missing in game?

FO3 didn't have a chinese faction (outside of the Anchorage DLC); although there were many hints about the chinese invasion lying around (books, weapons, armor).

I don't see how the Chinese invasion of Alaska correlates to them appearing as a faction in Boston. Doesn't sound very reasonable in the slightest. Hints at Chinese spies? Perhaps, they were all over Fallout 3. But an actual Chinese faction? There were no known full frontal Chinese invasions on the east coast of the US, let alone the main land of the country as a whole besides Alaska if I recall correctly.

The only known Chinese faction in Fallout is the Shi of Fallout 2, whose ancestors came from a beached submarine in San Francisco from after the Great War.

Their shown in the intro para-trooping which means attacking so it must be connected to the rumored Chinese invasion of Boston. In the fallout universe the Chinese have long wanted to crush America under their boot, so maybe this it their time to do it in an upcoming dlc like with anchorage?

Pretty sure they were landing in Anchorage, Alaska. Considering the snow and the large mountains of that scale that are nonexistent in Boston. The only mentioned place in lore of the Chinese invading in the US was Anchorage, Alaska.

China never invaded the USA outside of Alaska, which the US drove them out of by the time the nukes fell.

The US however was invading mainland China and making massive progress towards Beijing.