Licensing on VM with multiple instances and different versions

Hi,

I have a 2-proc, single-core VM with 2 sql server 2008 r2  (std edt) instances covered with a 2 proc sqlserver license for 2008 r2.
This VM is lightly used.

I now want to install a new instance on this VM, this time a sqlserver 2012 (std edt).

Will a new 4 vCPU 2012 license cover all 3 instances or how is the best (cheapest) way to license such a setup ?
(I have read that 4 cores is minimum for a 2012 license)

TIA,