I intend to change propellers from my Mini 3 and I would like to know if the screws in the bags have a security thread lock on the ends of the screws? Anybody know? I need to know, because if it doesn't come with a security thread lock, I intend to buy it and apply it to the screws before threading them.

Yes,  they have thread lock pre applied to the screws.  Always use new screws when swapping the props.  

No need to purchase thread lock as all DJI props come with replacement screws with thread lock already on them.

Hi there. Thank you for reaching out. (To back up on DAFlys and DowntownRDB) Yes, the screws should already have a thread-locking fluid pre-applied, and it is recommended to use the newer screws since the thread locker will wear off after removing the old screws. Thank your support and have a great day ahead.