Depends on the meet. Contact the meet director and see how he is running the show. Some meets are huge and certain weight classes go on certain days, some meets are smaller and can do everyone in one day.
If it's a small meet, everyone will do their first attempts before they cycle back to the second attempts. It's not based on weight class, but weight on the bar. If you're the strongest 100lber in the world and open with an 1200lb squat, you'll be the last guy to squat for openers. After all 3 attempts, then it moves on to bench.