ceteriz

divde the 12 balls into 3 sets of 4. 6 P( E3 C6 ~# O: n/ E' C
weight any 2 sets.
: B+ V; X, N) ^" D( H5 S& _' }( P5.39.217.76if the 2 sets are equal, then you know the heavy one is in the third
$ f2 t- s  @2 `+ p. N5.39.217.76if one of the 2 is heavier, then the heavy one is in that set
7 Q$ E2 L  L* b1 Q) J公仔箱論壇then divide the 4 balls into 2.
7 `& L' a; d! {& r6 v5.39.217.76weigh them
  a3 h+ K% M- R( R- a1 Z2 Zand divide the heavier into 1
) L8 p5 b; P- f6 D1 b% xand weight them.5.39.217.76* D! I1 O6 U2 d& J" U/ l! g" }% j
3 x weighing and get the ans.  
6 @# p+ N/ H- B, o. Y% V9 v5.39.217.76in less than 15 secs.