Maybe that changed at some point in the past couple decades, but it used to be that even in the middle of summer they wouldn't start the day with all 3 trains on all coasters unless it was Fri/Sat/Sun. Other midweek days, if it turned out to be busy that day, they'd pause the ride to add the 3rd train. They don't want to run all 3 unless they can keep them consistently near full (excluding a couple empty seats for lack of single riders), waste of mileage (i.e. maintenance costs over time).