[Sellize]

random(3) can be 0,1,2 (three options)
You need to use random(4), which can be 0,1,2,3 (four options)

Also, fixed your code.

PHP код:

if(bodypart == BODY_PART_HEAD)
{
    if(weaponid == 34)
    {
        rando = random(4);
        switch(rando)
        {
            case 0, 1, 2:
            {
                return 0;
            }
            case 3:
            {
                SetPlayerHealth(playerid, 0);
            }
        }
    }
}