A “3,5,7” is a “gut check” or endurance test. It refers to throwing a 3-punch combo, then a 5-punch combo, then a 7-punch combo, and repeating this nonstop for an entire round (how long that round lasts depends on your current level). This is an awesome anaerobic exercise and it trains you to mix up the power, speed, and angles of your punches to adjust for fatigue. It's also a great way to figure out how you like to punch.