i try to setting course lock for  my phantom 3 , but i'm fail. i need to help tq.

Hello Eddy, first you need to enable Multiple flight mode in MC settings. Then please fly the drone in the air under P-GPS mode and switch it to F mode. Hope these pictures could help you:

And practice in the sim, you want to be very familiar with getting in and out of this mode, as well as what controls do in that mode. I have found the controls not to be as intuitive as I expected, so practicing and familiarity with it is quite important.

tq,all my friend.