This is a brief overview of what you need to do:
* Make sure only 1 router is setup to do DHCP/Firewall/etc (We'll call this the Main router)
* Other routers are connected LAN port to LAN port of Main router (Do not connect using WAN/Internet port)
* Setup SSID name, channel, security to be identical on all 3 AP/routers
Assuming you're using your ISP provided modem/Router as the main and using a 192.168.1.x IP for LAN you'll have it set up as you would if that was the only router you were using. Setup both WAN and LAN on this one. An Example of your LAN setting might be:
LAN IP: 192.168.1.1
Subnet: 255.255.255.0
DHCP range: 192.168.1.100 - 192.168.1.250
On ASUS router:
WAN/INTERNET Port: Disabled
Disable Firewall, etc
LAN IP: 192.168.1.2
Subnet: 255.255.255.0
Gateway: 192.168.1.1
Connect to Main Router via LAN port to LAN port
On Dlink router:
WAN/INTERNET Port: Disabled
Disable Firewall, etc
LAN IP: 192.168.1.3
Subnet: 255.255.255.0
Gateway: 192.168.1.1
Connect to Main Router via LAN port to LAN port
Just remember when setting this up use a network cable to the router directly and once you disable DHCP on the router you will need to configure your network adapter to use a Static IP to reach them (initially). I personally configure the Secondary routers first (setup their LAN IPs, disable Firewall, WAN) but leave setting up the SSID til the end. Once the 3 routers are configured and connected via LAN ports you will be able to reach the configuration page of all three routers via their IP address and configure the SSID then.