As EP said above, it's hard to do these types of problems without an image to see. So, let's make an image.
He starts out with the dice, so he has a chance of 1-6.
When he rolls a 1 or a 2, he goes to the coin. Otherwise, he stays on the dice.
Now we can write which corresponds to what Bill's next turn will result in coin or dice
Now that there's an image, we can count how many chances go to the coin on that last turn.
There are 14 total outcomes by the 3rd move. 5 of them lead to using the coin on the last turn.
The probability of Billy using a coin on the last turn should be 5/14.
I don't know if this is right, since EP had 2 totally different answers than I got, so... xD