Zend certified PHP/Magento developer

How to setup squid proxy with multiple 3g modems?

I have 3 USB dongles on my computer. How do I setup squid to proxy each port to the appropriate dongle?

So I will have 3 virtual networks, with default gateways. I’m looking for squid directive to proxy

3128 -> x.x.x.x(first gateway)
3129 -> y.y.y.y(second gateway)
3130 -> z.z.z.z(third gateway)

I found config for 3proxy

auth strong
users login:password
users login1:password1
users login2:password2
users login3:password3
allow login,login1
socks -n -a -p3281 -i192.168.0.2 -e192.168.1.2
proxy -n -a -p3282 -i192.168.0.2 -e192.168.1.2
flush

allow login2,login3
socks -n -a -p3283 -i192.168.0.2 -e192.168.1.3
proxy -n -a -p3284 -i192.168.0.2 -e192.168.1.3
flush

But how to do it in squid?