Use my anti virus for more than one pc

I just bought the avg internet security 3 pc 
I use it only for one  pc ...but when I try to install it in my son pc its says its already used
what should I do    I just bought it  from 3 weeks  
 

Hello Ashraf,

I will help you with the installation.

I'm unable to find any active subscription of AVG with this email address.

I have sent you an email, please reply to that email with the requested information to proceed further.

Hello Ashraf,

I will help you with the installation.

I'm unable to find any active subscription of AVG with this email address.

I have sent you an email, please reply to that email with the requested information to proceed further.