It actually can be done in just two weighings.
(1) Divide the coins into three groups (A, B, and C) of three coins each.Weigh A against B. If neither group is heavier, rename group C "D"; if A or B is heavier, rename that group "D"Go to the next weighing.(2) Weigh two coins from group D against each other. If either is heavier, that one is the counterfeit; if they weigh the same, then it is the third coin in group D that is the counterfeit.
Hope this helps!