ABC, one of them is different weight than the other two (either more or less)
Possible ways to weigh them:
A v B
A not enough information
E C is odd one out
B not enough information
A v C
A not enough information
E B is odd one out
C not enough information
B v C
B not enough information
E A is odd one out
C not enough information
AB v C
AB not enough information
E C is odd one out
C C is odd one out
AC v B
AC not enough information
E B is odd one out
B B is odd one out
BC v A
BC not enough information
E A is odd one out
A A is odd one out
It's easier than the 12-coin one different puzzle, but introduces the ternary concept well enough to get you started on the harder problem (there are some other complications to it as well).
Weigh 2 coins. If the scales move you've found the lighter coin. If the scales balance, it's the third coin you didn't weigh.