Why my phone battery health is going down with in 3 months

Because All Batteries, ALL, with every full cycle charge, lose their ability to hold the same charge as new, little by little. Batteries are consumable parts and Battery Health will continue to decline until the time comes to replace the battery. This is normal and expected and there is no way to stop batteries from doing this.

Battery Health is an inexact figure that varies with usage. You need not worry until it reaches 80%, at which time you will need to make an appointment at the nearest Apple store for Battery replacement. In the meantime follow the advice in the link Carolyn provided.

Hi,

Help to maximize the battery > iPhone Battery and Performance - Apple Support