Last tweet of Captain Sim informs their followers the 777-300 and 777-300ER preview is online. Different airlines liveries are visible on this page.
It will be an expansion for the 777-200 base package, currently in public Beta test version 0.9. After various tests, I’d recommend you to wait for the final product to be released as it lacks too many and important features to enjoy it. Don’t expect high ratings if you use black boxes or flight loggers (like FS Flight Keeper). Just to name the worst missing features in the beta, Primary Flight Display and Navigation Display are not working correctly, no barometer or altimeter manual settings possible, autopilot and autothrottle faulty controls, erratic behaviour of electronic checklists, flight performance still needs important tweaks even with manual control input.