Replaced battery, starter, and alternator it still wont start.what could it be

When you say it won't start, do you mean the engine will not crank at all? Or does it crank without starting? If it doesn't crank at all and you have a known good battery in the vehicle, and can verify the starter motor and cabling is all good, I'd check to make sure the engine even turns manually with a breaker bar and socket on the crankshaft. Beyond that, hands on testing is needed to find out what's going on. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops