Loading different website with www and without www in url

[lonerunner]

I have problem in loading websites on ispconfig3. I have 3 different websites on my server but problem is when i want to browse websites if i type in url for example

www.firstsite.om or just firstsite.om it goes to my fist website which is main website on admin account.

But when i type www.secondsite.om it goes to second site but if i try to type secondsite.om it goes to firstsite.om

Same thing happens on third website.

Basically when i type www in url it goes to proper website but when i browse without www in url it always goes to first website which is default website of server on admin account.