~3 hours is plenty, presuming an on-site arrival. The only catch is that 7am is peak hour at SYD international - the curfew ends at 6am, and after that multiple large jets land in quick succession - in the hour before your flight arrives there's about 10 wide-bodies landing, all of which need to go through immigration/customs.

However given you're in Thai F you'll get an Express Pass which will speed your trip through both immigration and customs, so you should be through faster than most people even as a non-citizen.

The Virgin bus goes via land-side roads (ie, the same roads a taxi would take) so it's a little slower than the Qantas bus which runs airside, and can get held up by traffic - but regardless ~3 hours is plenty.