Domain Rediraction

Hi:
I am new to asp.net, I designed 3 different sites with their own domain name, I was not provided a way to host these different sites so I bout a developer package that could accommodate all the three sites at once, there is a slight problem how ever. I do not know how to write a redirecting script that I should put on the root folder. Assume the sites are 'www.site-a.com', www.site-b.com' & 'www.site-c.com'. how do I write the script to to see any of these site when I type in the corresponding domain name?
Please some one help me as I am very new to asp.net