Addon domains showing main domain!!

I have 3 web sites. i have my main site say mainsite.com i then added on 2 more sites as addon domains. the sites are totaly seperate and unrelated to each other except for the fact that they are addons. each site has a seperate wp install on it. i have got the home page of both addon sites to show as only addonsite.com but when you click on another page within the site it shows the main domain in ulr for example mainsite.com/addonsite.com/page120….ect thought i had it sorted but perhaps not!!!!!