My battery health is decreasing

Batteries are meant to degrade over time, which means after a while they won't hold the same battery capacity of which they used to.

You might also find this helpful as it provides you with useful information on maximising your battery.

I have iphone se 2020 and the battery is decrasing i installed ios 14.8 and battery is at 97% and the phone is only 2-3 months old

That would be about right although battery health is based upon a lot of variables, is an approximation, and is not linear.