Jak and Strike don't get an AoO because the goblin has only charged into their threat zone. Similarly for Jack (whose threat zone is a ring 2 tiles away).
If the goblin charges through the zone, or tries to get out of the zone, he provokes an AoO.
Diagram time.
___________
++?++++++g+
++U++++++++
++@++++++++
___________
? - Jakkara
U - Shadowstrike
@ - Jack
g - Goblin
The goblin, Jakkara and Shadowstrike are all armed with weapons that give them a 5ft range. This means they can threaten any directly adjacent square. The diagram below highlights the goblin's threat range with '.'
___________
++?+++++.g.
++U+++++...
++@++++++++
___________
Jack Rahl has a longspear that gives him 10ft range, which means he threatens a ring of space two squares away from him. This is highlighted by ',' in the below diagram.
___________
+,,,+++++g+
,+U+,++++++
,+@+,++++++
___________
In the first round of this sample movement, the goblin charges up to Jakkara as follows:
___________
++?g<----++
++U++++++++
++@++++++++
___________
None of the three adventurers get AoO because although the goblin has entered the threat range of all three (take another look at Jack's threat ring if you are not sure), he is not moving within that range.
Say the goblin moves one step south, putting him in front of Shadowstrike.
___________
++?++++++++
++Ug+++++++
++@++++++++
___________
He has moved through the threat range of Jakkara and Shadowstrike (both starting and ending within one square of them), provoking AoO from both of them. He has also moved out of Jack Rahl's threat range by coming within the ring of threat. This provokes an AoO from Jack Rahl.
Say all three miss or the goblin is tough enough to take the three AoOs. He moves south another step, putting him in front of Jack.
___________
++?++++++++
++U++++++++
++@g+++++++
___________
The goblin has moved out of Jakkara's threat range. If Jakkara had the Opportunist feat and could make multiple AoOs, he would get another AoO for this. The goblin has also moved through Shadowstrike's threat range (as he is still within a square of him). This would prompt another AoO if Shadowstrike was an Opportunist with multiple AoO attacks.
The goblin has moved inside Jack Rahl's threat range, however, and as such does not provoke an AoO from Jack because he cannot strike effectively at targets within 5ft using the longspear.