interesting (?) river decision in 3-way pot

[neon]

This is my first post here; I regularly read the analysis of the posters in this forum, and have learned a great deal from you all in just a few short months. I've only been playing poker for about eight months, but have been pretty successful as of late at 2-4 and 1-2 NL ring games online and feel like I have a pretty solid understanding of the game for someone who's only been playing a few months. I hope some of you find this hand somewhat interesting and worthy of comment. If not, I apologize.

Anyway, six-handed 2-4 NL online. I'm in the BB, two villains in this hand are SB (~550) and CO (~225). I have them both covered. SB is an uber-agressive, somewhat tricky player; I've seen him make huge overbets with the nuts, 12XBB preflop raises with A7o from the BB, etc. CO is a pretty ABC TAG who has been sitting for about an hour and has made maybe three raises preflop.

Folded around to CO, who makes it ~15 to go (I accidentally deleted the hand history, so I'm going from memory here). Button folds, SB calls, I call w/ K [img]/images/graemlins/club.gif[/img]J [img]/images/graemlins/club.gif[/img]. Flop is J 9 3 rainbow, one club. Check, Check, CO bets 35, call, call. turn is the 4 [img]/images/graemlins/club.gif[/img]. SB springs to life and bets 75 into a ~150 dollar pot. I think for a minute and call, CO insta-pushes for another 100 or so. SB folds (?).

So there is now 470 in the pot with 100 left for me to call. I am 99 percent sure at this point that CO has either aces or kings.

Is this an automatic call?

Thanks in advance.

-neon.