Just use the below code
string ToMail="you1@gmail.com,you2@gmail.com,you3@gmail.com";
System.Net.Mail.MailMessage mail = new System.Net.Mail.MailMessage();
mail.To.Add(ToMail);
The e-mail addresses to add to the System.Net.Mail.MailAddressCollection.
Multiple e-mail addresses must be separated with a comma character (",").
Regards,
Manoj