Battery health dropped by 1% in 3 months

That's actually pretty remarkable. Most users can expect battery health to decrease 1-2 percent each month.

Batteries are consumable and begin to degrade immediately. Yours is designed to hold 80 percent capacity at 500 charging cycles. Learn more at https://support.apple.com/en-us/HT208387

What's the problem? Apple rates iPhone batteries for a typical 80% battery health after 500 complete charge-discharge cycles. Granted, everything is simplified into a model where there's a lot of variation, but 1% after 3 months sound normal. I've heard of better than that, but I've certainly heard of worse. Batteries also have manufacturing variations, where I've heard of some that quickly updated to 98-99% while others might have even reported more than 100% (where the value is just the rated value).

Batteries are wear items. They will inevitably decline in battery health.