Question 725450
First make a probability tree of all the possible outcomes (given the restriction rules)

<img src="http://i150.photobucket.com/albums/s91/jim_thompson5910/Algebra%20dot%20com/1-1_zps1c074b72.png">


The values in red are the probabilities for each scenario. For example, there’s a 12.5% chance of flipping THH (start at the top and go left, right, right)


Circle all the probabilities where T shows up 3 times to get this


<img src="http://i150.photobucket.com/albums/s91/jim_thompson5910/Algebra%20dot%20com/2-3_zpsf2df3573.png">


Add up all those circled probabilities: 12.5+6.25+6.25+6.25 = 31.25


So the probability of that the third tail does occur is a <font color="red">31.25%</font> chance, which in decimal form is <font color="red">0.3125</font>