Ajax Mutiple Requests Handling Problem

Hi, I am developing my site, [login to view URL] I am new to Ajax, and I encountered a mutiple requests handling problem that I do not know how to solve, For example, if I typed in a domain, [login to view URL], pushed "enter" button to check the page rank on my site, it will send 3 requests serperatedly to the server, you can see the 3 "Loading images" if I quickly re-push "enter" button, it will resend those 3 requests, however, the problem is, sometimes, I got 3, sometimes, I only got 2 returned results, sometimes, 1 result, sometimes, none. I know this gonna be something wrong with mutiple requests handle, but I do not know how to solve it, I tried to add abort() function to end all my 3 requests every time onclick() being called, but it still doesn't work, help wanted to get this thing work correctly. thanks.