crankshaft sensor and cyl #3 misfire HELP!!!!

[HTX86chevy]

My truck is acting up..... it takes about 2-4 tries to get it to crank up and run.... aslo when I press the gas to move forward or reverse it is acting up like it is deprived of GAS. I took it to the autoparts so they can scan the codes and they said:
1.) 2 front o2 sensors need to be changed cuz its running too lean
2.)crankshaft sensor is bad
3.) cylinder #3 misfire

I recently changed 1 front o2 sensors about 3 weeks ago should i also change the other?
where is the crank sensor located anyone have a pic???
what would cause misfire on one specific cyl?????? anyone have the firing order so i can identify the #3 cylinder?

oh yeah, its a 2001 gmc sierra 1500 4.8 automatic
i also changed all 8 spark plugs 3 weeks ago but didnt change the wires cuz they were too expensive lol