What does "Battery Cycle Count - 3" mean on iPhone?

That’s a new feature in iOS 17.4 on the iPhone 15 series only. An iPhone battery is guaranteed to stay above 80% maximum capacity for at least 500 “full charge cycles”. That works out to about 1% loss of capacity for every 25 full charge cycles. So if your battery cycle count is 3 that means 3 full charge cycles.

A full charge cycle is 0-100%, or any combination that adds up to 100%, such as 50% to 100% twice, 25% to 50% four times, etc. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.