That “Default contact” is the one that only lets you set one mail address.
I don’t know for sure if that contact field supports using local mail aliases, but if it does and you use sendmail as an alert transport you could setup a mail alias that goes to your three email addresses.
E.g. in /etc/aliases you could have:
outage_notices: [email protected], [email protected], [email protected]
And in Default contact:
outage_notices
Or if you could create a mail alias with forwarding to those addresses on your mail service you could do basically the same thing.