Cant connect to my wifi (HELP)

You haven't told us which router and which ISP. Those two bits of data might be helpful.

In a situation where 2 out of 3 devices connects with "no problem" makes me inclined to "fix" the third device. Sometimes fixing involves debugging. I have found with OSX that simply doing an archive and install is the quickest fix. This sort of brute force approach on a Windows box might require days of effort but on OSX, it is a process that can run mostly unattended. You simply boot from the OS install DVD or USB stick or recovery partition and pick install (on newer versions of OSX) or archive and install (on older versions of OSX). Of course it is always important to have a backup. I recommend using Time Machine to a local USB or Firewire disk along with another method such as Carbon Copy Cloner or Crashplan.