All three coins will be at (13,7).

The key to solving this is to compute the x and y coordinates separately. We are given the first three x coordinates, 0, 3, 9 and each subsequent x coordinate will be 3/4 of the previous two less 1/2 of the one before that. Eventually this settles on an x coordinate of 13. Doing the same for the y coordinates will eventually settle on 7.

In general, given starting positions of A, B, C, the coins will converge on: (-2A+B+4C)/3.