KK HU vs 3 better

[theweakestlink]

i dont have a read cuz I just sat down at the table. most players at this limit aren't sophisticated enough to three-bet with mediocre hands and are also fairly passive, so I figure that most unknowns will have pretty good hands to three-bet with preflop.
Honestly I felt that he either had a set or an underpair when he raised me on the flop.. but I'm still pretty inexperienced so i wasn't sure
Comments are appreciated.. thx!
PokerStars $0.05/$0.10 Limit Hold'em - 8 players
[url=http://www.deucescracked.com/Hand-Converter.html]Hand
Preflop: Hero is CO with K[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/diamond.gif[/img]
UTG folds, UTG+1 folds, MP1 folds, MP2 folds, <font color="#FF0000">Hero raises</font>, Button folds, <font color="#FF0000">SB 3-bets</font>, BB folds, <font color="#FF0000">Hero caps!</font>, SB calls.

Flop: (9.00 SB) 9[img]/images/graemlins/club.gif[/img], T[img]/images/graemlins/club.gif[/img], 3[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#FF0000">SB bets</font>, <font color="#FF0000">Hero raises</font>, <font color="#FF0000">SB 3-bets</font>, Hero calls.

Turn: (7.50 BB) 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#FF0000">SB bets</font>, <font color="#FF0000">Hero raises</font>, <font color="#FF0000">SB 3-bets</font>, Hero calls.

River: (13.50 BB) 7[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#FF0000">SB bets</font>, Hero calls.

Final Pot: 15.50 BB