The "wired" is relevant only to how the camera is powered, not to how the device transfers its video data feed which is done via WiFi connection.
The power wire is permanently attached into the stand at the camera end and has a male USB A-type connector at the other end of the 3m cable - this can be plugged into a mains wall outlet using the supplied USB power adapter. I have also successfully powered one of my cameras by plugging it into a standard computer USB A-type port, but not sure if LogiTech officially supports this.
As to the number of cameras supported, I currently only have 2 so cannot confirm first-hand that 3 or more still works fine. But the LogiTech website lists a "Multi-Pack" (PN:961-000474) which includes "2 Wire-Free Cameras + 1 Wired Camera", so I think it's safe to assume more than 2 is possible! Additionally, in the FAQ section of their website, LogiTech goes further by stating that there is "no limit to the number of cameras you can associate to your account or mobile device". However, LogiTech continues by noting that "Image quality may be affected by the number of cameras on the same network, depending on your current internet speed."