I'm going to take it apart because when I change the 3rd, it doesn't take it, it's my turn from 2nd to 4th, the third one also skates where the air filter goes when I changed it, it's wet with oil
Ok, what I recommend is to take as many photos as possible before disassembling something.
What makes you wet the filter can be from closed intake valve, or valve guides or valve seals (on some sides they are called cups) going through worn or broken rings.
When you disassemble to see the problem of third look at the selector or it can be a worn cam so you say it would not be the gear because if you would find bits of metal when you change the oil, comment that you are watching to be able to orient yourself. how to disassemble and assemble motorcycle boxes.