Rathena already has one available.
The same script from the SVN (LINK) can be found inside your npc/custom folder just enable it under your scrips_custom.conf
Inside the file there is this part
OnInit:
setarray .Rebirth[0],99,50; // Minimum base level, job level to rebirth OR change to third class
setarray .JobReq[0],10,40; // Minimum job level to turn into 1st class, 2nd class
set .ThirdClass,1; // Enable third classes? (1: yes / 0: no)
set .SNovice,45; // Minimum base level to turn into Super Novice
set .LastJob,1; // Enforce linear class changes? (1: yes / 0: no)
set .SkillPointCheck,1; // Force player to use up all skill points? (1: yes / 0: no)
set .Platinum,1; // Get platinum skills automatically? (1: yes / 0: no)
end;
On the last part. The settings are already for 3rd class.