If you have installed the AX2012-B and AX2012-C Virtual machines, then not used the B or C ones for over a month, when you log into them again you will probably get the error "The trust relationship between this workstation and the primary domain failed."
This is because the link between the B (and/or C) and the A machines from a domain trust point of view must be done within 30 days, or it expires.
The solution is actually quite simple. Using B as an example - as this is what happened to me, when I went onto B to install Management Reporter 2012:
First - if you logged out of B before you last saved it, you won't be able to sign on. So change your network settings for that VM so that it doesn't use the private "AX2012" network adapter. Now you can Connect and log in. Change the settings back to use the private network (or you won't be able to connect to AX2012-A). The good thing is you can do this while the VM is up and running, and you're logged in, without any issues.
Once logged in to B, go into your computer Properties, and in your Computer name, domain and workgroup settings area, click Change settings. You will see it is in the domain: contoso.com. Click Change, then move it to a workgroup, DO NOT REBOOT, then move it back to the contoso.com domain. It will (naturally) ask you for an Administrator user and password on that domain. Enter that, reboot, and all is now fine - you can now remote desktop to this VM.