if i am in third and i slow down doi put it in nutral and if i slow enough then shift to 2nd?

well you can do either.

 

Just pull it out of gear, coast and use your brakes.

 

You could also hit your brakes in 3rd to slow down a little, clutch in, shift to 2nd and slowly let out the clutch. Don't just throw it in 2nd from 3rd at 50 MPH That's why I say let the clutch out slowly. If your RPMs start coming WAY up, and fast, befor the clutch is even out all teh way; then you know it was too early to go into 2nd and you need to clutch back in and use your brake.