Want to use multiple email addresses for one contact
It seems like you would need to enter each of the email one at a time. For contact NAME with EMAIL1, EMAIL2 and EMAIL3, you can type NAME in the to: line and Mail will give you the options to choose which email. 

For all three you will need to type NAME three times. Although on the to: line it will show NAME three times, if you click on the little arrow next to the name it will show you which email each name refers to. Good luck.