Please help! How do I find out the firing order for my 1997 Plymouth Voyager with a 3.0 engine? Thanks!?

We have replaced, trans, muffler, plugs, wires, cap, rotor,crank sensor, netural safety switch, brakes, wheel cylinder, radiator, and air filter. Tonight we are doing the battery and the ignition switch. Still getting codes P0301 and P0306. So mechanic in town tried to sell me a new engine. NOT!