Hi,
You can add them into the current zone file for company.com
us IN A (IP address for us.company.com)
us IN MX (mail-server FQDN for mail to us.company.com)
europe IN A (IP address europe.company.com
europe IN MX (mail-server FQDN for mail to europe.company.com)
asia IN A (IP address for asia.company.com)
asia IN MX (mail-server FQDN for mail to asia.company.com)
If you want to create new zone files for each subdomain and your GTM is the master for the subdomain, create it as a master.
Don't forget to create the NS records in the parent zone as well as that will be queried during the resolution.