I wonder if installing 2 more (USB to Ethernet) NICs to my PC can be risky if I need that only my PC can access to the three networks, and no any other host in these networks... Could my PC act like a router, allowing traffic from any host to any network?
Simply adding NICs to a single PC doesn't make it a bridge or a router automatically. However, it would be pretty simple for someone with admin access to make the PC become a bridge or router between the networks. For example, someone could enable Internet Connection Sharing in Windows. Also, if someone guessed a username/password on the PC (or otherwise compromised it), they could then SSH or VNC into the PC and then from there connect to the other networks.