Don't do it with Squid : you need control for everything, not just for HTTP on port 80.
The answer requires iptables with the '--quota' option, which implements network quotas by decrementing a byte counter with each packet. The argument of "--quota" is a value in bytes.
There shall be one chain for each user. First rule of the chain counts down a 13 GB quota for packets from 192.168.0.2
and accepts the packet if it is below quota:
iptables -A INPUT -p tcp -s 192.168.0.2 -m quota --quota 13958643712 -j ACCEPT
Second rule of the chain classify over-quota packets in a tc class of your choice :
iptables -A INPUT -p tcp -j CLASSIFY --set-class 1:12
Then it's all classic traffic shaping : http://tldp.org/HOWTO/Traffic-Control-HOWTO/
Of course, you need to use static IP allocation or make sure that DHCP allocates addresses fixed by device's MAC address - and you need to block all addresses but the identified ones of the devices belonging to one of the three users.
By the way, you mention that "when 2 people browse the internet they should get 1 Mbps each, and when 3 people access, they should get 2Mbps divided by 3" but you can do better than that when you set up your traffic classes hierarchy: your requirement should rather be "when two people browse the internet they should not get less that 1 Mbps each, and when three people access, they should get not get less that 2 Mbps divided by 3" so that each can get more if the other people use less than their guaranteed throughput... And tc lets you do that !
Since your router is supported by openwrt and dd-wrt, you have all the tools you need !
Most common routers have a statistics page where it shows the IP Address or Hostname of the device and how much traffic is going from and to the device.
You can use wireshark to see what the traffic is, e.g. torrents / streaming.
If they are personal laptops, they would most likely indicate there name within the hostname of the device on the routers webpage.
Now you know who is hogging all the bandwidth, your router may allow bandwidth management, if this is the case then just cap that device. If not you can use the web filter on the router and block access to links containing certain words as such. E.g. torrents, tracker, announce
Best Answer
100 Mbps is the maximum speed your LAN can support.
Your internet speed is 250 Kbps ( or max 750 kbps).
Regarding internet speed, you can test on websites like speedtest.net
Do remember that LAN and internet speed are two different things.
Following links may be helpful :