Wan Networks
eth0 192.168.1.10/24 gateway 192.168.1.1
eth1 192.168.2.10/24 gateway 192.168.2.1
eth2 192.168.3.10/24 gateway 192.168.3.1

Lan Network
eth3

edit /etc/iproute/rt_tables
201 table1
202 table2
203 table3

edit /etc/network/interfaces

auto eth0
iface eth0 inet static
        address 192.168.1.10
        netmask 255.255.255.0
#       gateway 192.168.1.1
        broadcast 192.168.1.255
        network 192.168.1.0

        post-up ip route add 192.168.1.0/24 dev eth0 src 192.168.1.10 table table1
        post-up ip route add default via 192.168.1.1 table table1
        post-up ip rule add from 192.168.1.10 table table1
        post-down ip rule del from 192.168.1.10 table table1

auto eth1
iface eth1 inet static
        address 192.168.2.10
        netmask 255.255.255.0
#       gateway 192.168.2.1
        broadcast 192.168.2.255
        network 192.168.2.0

        post-up ip route add 192.168.2.0/24 dev eth1 src 192.168.2.10 table table2
        post-up ip route add default via 192.168.2.1 table table2
        post-up ip rule add from 192.168.2.10 table table2
        post-down ip rule del from 192.168.2.10 table table2

auto eth2
iface eth2 inet static
        address 192.168.3.10
        netmask 255.255.255.0
#       gateway 192.168.3.1
        broadcast 192.168.3.255
        network 192.168.3.0

        post-up ip route add 192.168.3.0/24 dev eth2 src 192.168.3.10 table table3
        post-up ip route add default via 192.168.3.1 table table3
        post-up ip rule add from 192.168.3.10 table table3
        post-down ip rule del from 192.168.3.10 table table3

IPTABLES COMMAND
iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
iptables -t mangle -A PREROUTING -m mark ! --mark 0 -j ACCEPT
iptables -t mangle -A PREROUTING -j MARK --set-mark 10
iptables -t mangle -A PREROUTING -m statistic --mode random --probability 0.5 -j MARK --set-mark 20
iptables -t mangle -A PREROUTING -j CONNMARK --save-mark

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -t nat -A POSTROUTING -o eth2 -j MASQUERADE

edit /etc/rc.local
ip route add default scope global nexthop via 192.168.1.1 dev eth0 weight 1 nexthop via 192.168.2.1 dev eth1 weight 1 nexthop via 192.168.3.1 dev eth2 weight 1

edit /proc/sys/net/ipv4/ip_forward
1