Easy way to check...

From each of the machines, go to http://www.whatismyip.co.uk. It will tell you the IP the "internet" believes your connection is originating from. Then, go to each of your machines, go to Start -> Run and type in cmd, press return. At the command-prompt which should appear, type in the following to find out each computer's actual IP address: ipconfig

If the IP you see on WhatIsMyIP is different from the one you see using IPConfig, then it's likely that your router is the only IP that gets displayed to the internet. Additional to this, if IPConfig gives you an IP that starts with either 192.168.xxx.xxx or 172.(16-31).xxx.xxx or 10.anything, then your machines are on a private IP-address behind the router.

So, in most cases, the answer to your question of "Do all three of my PC's have individual IP's to identify each one, but when accessing the net they use the same gateway IP or something?" would be Yes......every single public-facing device on the internet has a unique IP address (if you are using private IP-addresses for your computers, the router would be your public-facing device - if this is the case, your gateway IP will be the router, which will also show up in IPConfig).

The PF logs showing multiple IPs means your connection is likely using a dynamic IP address system (ie. every time you connect to the internet, a different IP address is assigned to your router by your ISP, from a pool of available addresses). So, the logs would show quite a long list of IP addresses, as each time you connect to the internet, you'll be given a different IP address.

Unless post-timings show that both of you were online at the same time, I'd say it's just a case of you both using the same ISP, and you've had the massively-improbable "luck" of Canesten being assigned an old IP address you were once assigned in the past.

And, to answer another query, WAN = Wide (not Wireless) Area Network, it's just another name for the internet......LAN (Local Area Network) is anything your side of the router (hence "local"), WAN is anything the "other" side of the router (ie. through the phone-cable)