what is 3x etf and different timezone effect

Im fairly new to stocks and seen 3x etf Apple. this opens up at 8am Uk time and say it showed minus 15% what does this mean as apple stock then opens up at 2.30pm will the actual apple stock also dump 15% and then either up or down or becuase its a 3x etf it will dump 5% as soon as new york market opens thanks?

Also say 3x etf is in profit is it good to keep it open for days or weeks untill i feel like its time to sell thanks