minimum 3 max 6 (If you are splitting into 3 piles each time).

incase you don't know: "!=" means "not equal"

BEST CASE = 3 tries
1)if A=B then the coin is in C. Split C into three piles (again A,B,C)
2)if A=B then the coin is in C again. Split C into the three last coins.
3)if A=B then C is the coin.

WORST CASE = 6 tries
1)A!=B (piles of 9)
2)Now test A and C
if A=C then split B into ABC
if A!=C then split A into ABC
3)A!=B (piles of 3)
4)Now test A and C
if A=C then split B into ABC
if A!=C then split A into ABC
5)A!=B (last 3 coins)
6)Now test A and C
if A=C then B is the coin
if A!=C then A is the coin