My mobile purchased by 3 months but battery health is 99 what the reason?

99% battery health after 3 months is actually pretty good. Your battery health is fine and is undergoing normal usage. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. You should change out the battery once your Battery Health reaches below 80% which will take over 2 years to reach under normal usage. Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➔ iPhone Battery and Performance - Apple Support

Axel F.  

Read what your iPhone’s screen is displaying. Also read what I have posted.

After that, describe what you meant in the response above because I am not sure what you are attempting to communicate. Thank you.

Axel F.