Connecting 3 networks to 2 ethernet ports.

Apologies for what I assume is a noob question. I've been googling for about an hour and haven't yet found a satisfactory answer.

The problem I'm trying to solve:

We have an enterprise PC with 2 ethernet ports. This needs to communicate with 2 external networks that can't be connected together, and also needs to communicate via ethernet with test equipment that is physically nearby. This test equipment doesn't need to connect to the internet, or to anything at all besides the PC. It can't communicate or even see the 2 external networks, and vice versa. None of the three networks should see each other. The communication is roughly like this:

PC <---> Network 1 PC <---> Network 2 PC <---> Test Equipment 

I'm unclear on what specific path to take. Is this a job for VLANs? Everything I've read about VLANs seems to indicate it's useful for isolating groups of devices on the same network, but that doesn't seem to be quite what I need. Is a managed switch enough, or is a router needed?

I was thinking that a managed switch should be able to allow two devices to communicate with one another while hiding one of them from the outside world. Is that line of thinking correct?

Any help is appreciated! Thank you.