Microsoft Active Directory Migration From 2008 to 2012 Plan

Dears,

We have plan to migrate our Microsoft active directory 2008 to AD 2012, in our current environment. We have three domain controller one primary two secondary one of them is VM, my plan is to preparing three servers also with OS 2012 one physical two VM's, I wants to find Microsoft recommendation for best plan if you have three servers shall I make one physical and two VM's or all three VM's. Please if some one can send to me letter or recommendation by Microsoft which way is the best. Appreciate.

Thanks.