Hello AZM
I order to achieve such redundancy, you will need to use BGP in a dual or multi-homed configuration. Specifically, this means that the IP addresses of your web servers must be advertised via all three ISP connections, with varying attributes to indicate which is the primary, secondary, and so on, route to get to your servers.
Now there are several “administrative” issues involved here. If your public IP addresses are provided by your ISP, and you have different ISPs at each site, you may not be able to route IP addresses of one ISP via another ISP. If the public IP address range is your own, then you can advertise this freely however you like, assuming you are running eBGP between your equipment and each ISP’s equipment. Much of what you can and can’t do in this area has to do with the policies of each ISP, and for this reason, an extensive chat with them describing what you want to do will go a long way in finding the best solution.
In such a situation, controlling outbound traffic is easy. If an ISP fails, you route traffic via the secondary route. You have control over your own equipment to be able to reroute such traffic. The challenging part of incorporating such redundancy that you describe has to do with incoming traffic. Such a situation requires you to attempt to control incoming traffic, that is, the way that each ISP will route traffic that is destined for your servers.
The first rule about controlling inbound traffic is Attributes that you would use to that you do not have ultimate control over how traffic enters your BGP Autonomous System. All of your eBGP peers can override all of your attempts to influence incoming traffic. Having said that, however, there are four ways in which you can attempt to influence incoming traffic to achieve the redundancy you need: Leaking more specific routes, MED, AS-PATH prepending and Community/Local pref agreement.
You can find out more about each of these in Unit 3 of the BGP lessons. Here’s a link to the attributes lesson:
Also, you can find out about the dual and multihoming scenarios at this lesson.
I hope this has been helpful!
Laz