i replaced the battery,alternator and starter but battery is not charging. why?

Between the battery and alternator is a link called the fusible link. It is usually located near or on the side of the main fuse panel. Follow the smaller red wire from the battery or the wire coming off the alternator to this link. If it is burnt out you will need to replace it. The way to test for this prob. is with a multimeter. Touch red to red and black to black on the battery and get your read. Then touch black to black on battery and red to the wire from the alternator anywhere on the other side of the fusible link. If the reading matches the battery the link is ok. If it is zero the link is burnt out and needs replaced. The vechicle is turned off through the whole process. Otherwise, make sure all wire and battery leads are hooked back properly to all components.