computer A: 192.168.1.3 (eth0)

computer B: 192.168.1.7 (eth0), 192.168.1.5 (eth1), 192.168.1.6 (eth2)

sudo sudo ip route del
sudo ip route add default via 192.168.1.6

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.6     0.0.0.0         UG    0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth0

#!/bin/bash

modprobe nf_conntrack
modprobe nf_conntrack_ipv4
modprobe nf_nat
modprobe iptable_nat

IF="eth2"
ADDR="192.168.1.3"

sudo bash -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'
sudo bash -c 'echo 1 > /proc/sys/net/ipv4/ip_dynaddr'

sudo iptables -P INPUT ACCEPT
sudo iptables -F INPUT
sudo iptables -P OUTPUT ACCEPT
sudo iptables -F OUTPUT
sudo iptables -P FORWARD DROP
sudo iptables -F FORWARD
sudo iptables -t nat -F
sudo iptables -A FORWARD -i $IF -o $IF --source $ADDR -j ACCEPT
sudo iptables -A FORWARD -i $IF -o $IF --destination $ADDR -m state --state ESTABLISHED,RELATED -j ACCEPT
sudo iptables -t nat -A POSTROUTING -s $ADDR -j MASQUERADE