My 2002 Nissan Maxima take a long time to start up. It turns over for about 2-3 seconds, then starts up. I don't know if it's the crank sensor or the front or rear cam sensor.

You have to start somewhere. See if the engine is still getting gas and spark. Do you hear the fuel pump turn on for 2 seconds when you turn the key to on, not start? It should turn on to prime the fuel line. If unsure, use some starting fluid, a shot or two into the air intake, and then crank the engine. If it rumbles a bit and now tries to start, then you know was a fueling problem. Have the fuel pump checked or replaced.
If the fluid makes no difference, see if you still have spark on one of the spark plug wires. If no spark, the ignition system must be diagnosed (ignition coils, ignition control module, ecu, crank and cam position sensors.)

Check for any loose wire harness connection or bad contacts.

A crankshaft position sensor or a camshaft position sensor can both cause a no fire condition. Both of these sensors are magnetic sensors that communicate with each other and pass that information on to the computer, letting it know that the motor is in correct time. Of either of these sensors go out, the computer doesn't know if the motor is in time, and can keep the engine from firing.

If you just replaced the rear main seal, double check to make sure that both sensors got reconnected (if you had to disconnect them)first thing. After that, you might look into testing them. I'm sure you can do an ohms resistance test, but also make sure you're getting voltage from the connector while trying to crank the motor. Ican tell you from experience that those cars have issues with their camshaft position sensors. Good luck!

Could be crankshaft sensor.maybe needs tune up,fuel filter.

Sounds like a failing crank or cam sensor; does the car have spark? Remove the main lead from the coil and position near a grounded piece of clean metal, place it @ 1/4" away and crank engine over for 2-3 seconds and watch for spark; if nothing it will give you some direction. If there is spark check for fuel injector pulse; rent some NOID lights from Autozone or such and unplug one injector and plug in NOID light and crank over for 3-4 seconds if you get a pulse of light the injector pulse should be ok. Next area I'd check is fuel pump, you'll need to check for fuel pressure, some special tools needed here plus your working with gasoline, take care of no spark/no flame. You may be able to listen for your fuel pump...turn off items such as fan, radio and such, make it quiet crank over motor for 2-3 seconds listen for a click (that would be your fuel pump relay and when you let go of your key you should hear the fuel pump run for just a brief second, I expect pump in the tank.
One of these I hope should get you in the proper direction.
I hope this may have helped you,
Tom

Check timing belt. Shoot a timing light down at your crank. If the timing mark jumps around bad, your belt has slipped.

Thoroughly clean interior of throttle body especially area where butterfly flap seats and all drillings, use aerosol carb cleaner, check air filter and spark plugs (platinum type only!). Then set idle with a consult unit or similar, engine management must be in service mode to set idle properly.
Also test coolant temp sensor it may be open circuit. If you don't know how to do this ask me.
Forget about earth if starter turns engine over at normal speed.

My car was having the problem I had my ingnition switch change, stater and a new battery and now it's fine.

if vehicle hesitates to start do you mean long cranking time (posible fuel pump) or no crank at all then suddenly starts then it is possible starter solenoid (notorious) - complete starter assembly needs replacement.

Test for proper fuel pressure with a test gauge on the fuel rail, most throttle body injection cars (TBI) are between 13 psi and 17 psi. and most (DPI) direct port inject systems are between 40 psi and 55 psi. If no or little fuel pressure is observed and there is 12 volt power present at the fuel pump, the fuel pump has failed and needs replacing.