For each router, download the OpenWRT image from downloads.openwrt.org, log into the router, and upload as a firmware update. The router is now running OpenWRT.

Next, log in over telnet (telnet 192.168.1.1) after getting an IP, and set a root password using passwd.

Log back out, and log in using SSH, using the password just set.

Edit /etc/config/wireless, and replace the config wifi-iface block with:

config wifi-iface
   option encryption 'none'
   option device 'radio0'
   option mode 'mesh'
   option mesh_id 'mesh'
   option network 'mesh'

Use the UCI tool to bring up the wireless interface (uci set wireless.radio0.disabled=0; uci commit wireless; wifi)

Repeat the above steps for the other 2 routers.

For router #1, add a new interface to /etc/config/network, named 'mesh':

config interface 'mesh'
   option ifname 'wlan0'
   option proto 'static'
   option ipaddr '10.0.0.1'
   option netmask '255.0.0.0'
   option ip6assign '60'

Also in router #1, edit /etc/config/dhcp, to allow DHCP leases on the new interface:

config dhcp 'mesh'
   option interface 'mesh'
   option start '2'
   option limit 16777213
   option leasetime '24h'
   option dhcpv6 'server'
   option ra 'server'

In routers #2 and #3, add an interface to /etc/config/network, but this time, change it to this:

config interface 'mesh'
   option ifname 'wlan0'
   option proto 'dhcp'

Also in routers #2 and #3, add a DHCP ignore rule:

config dhcp 'mesh'
   option interface 'mesh'
   option ignore '1'