What else can make my vehicle not start? Changed the batt and alt. and starter

Does the engine turn at all with a breaker bar manually? Ensure it turns before anything else. Ensure a known good battery fully charged and able to handle a load is installed. Make sure battery cables are good, free of corrosion and mounted directly to the battery -- no adaptors of any kind. If all this is good, then you could have a starter concern. Further testing is needed.