I'm a little confused as to how to set this up and need a little guidance.

What i'm trying to do is create a lab environment on my hyper-v host that consists of 3 subnets: 192.168.1.0, 192.168.2.0 and 192.168.3.0. I created a server 2012 VM with RRAS and attached 3 NICs to it with the following:

NIC1: IP = 192.168.1.254, SM = 255.255.255.0, No Gateway or DNS servers.

NIC 2: 192.168.2.254, SM = 255.255.255.0, No Gateway or DNS servers.

NIC 3: 192.168.3.254, SM = 255.255.255.0, No Gateway or DNS servers.

So far so good, I create 2 additional VMs on the 192.168.1.0 network, one is a DC/DNS/DHCP and the other is an SCCM server. However, I need to also have internet connection for these lab VMs. My hyper-v host has 2 NICs with access to an internet connection. How do I go about this considering that I want to keep this lab isolated using those subnets but at the same time having internet connection?

Thanks,

Configure a server with at least two NICs (can be physical or virtual) with RRAS.  One NIC is connected to the internet and the other to another network to which you want to give internet access.  The machines that are not on the internet network would point to the RRAS server's non-internet network address as their gateway.  Obviously a valid DNS is needed in there - that's obtainable in a lot of different ways.

 All I would add to Tim's post is that it is no different from running a domain with physical machines  on a physical switch and using a physical router to give it Internet access. The way you run the domain is just the same. How you configure DNS is just the same.

It would all be pretty pointless if the virtual lab didn't work just like a physical one.