Why does my iPhone 8 drain battery 3 times a day with normal use?

Go to Settings > Battery > Battery Health and see what is stated next to Maximum Capacity. It could be that your capacity has reduced, which will result in fewer hours of usage between charges. Maximum Capacity normally reduces over time and the only way to resolve this is to replace the battery.

Here’s battery replacement info, including cost→iPhone Battery Replacement - Official Apple Support. An Apple store can usually replace the battery while you wait.

Are you charging your battery overnight?

That is actually very good for a 4.5 year old iPhone. You don’t really need to worry about that reading until it gets down to 80%.

Some steps you can take to slow down this process:

* turn on optimized battery charging

* plug your phone into power and let it charge overnight every night

* avoid force closing apps unless they are missbehaving