New alternator and a new starter and a new battery in my 99 Cadillac deville northstar and the car won't start or turn over what is the problem with the car ?

Maybe a problem with your security system? Even so, with testing, you should be able to see if starter motor will crank the engine, just to see if it will? That doesn't mean it will start and ron. At least you know the starter will crank the engine and the engine isn't seized? Make sure battery has a full charge and battery connections all good, at all ends, not just at the battery. At the starter motor, two voltage circuits, one comes from battery and hot all the time. The other circuit goes hot when theft relay is activated. The starter motor should ground through engine block. Voltage and ground circuits both are important. Any testing at your starter motor make sure transmission is in park or neutral and parking brake is set. There are other components involved with starter circuit, neutral switch--ignition switch--as well as wiring circuits--security system and pcm. I am on computer at library so there are some things I can and can't do on this computer, or I just don't know how. good luck