The sample space is not just {heads,tails}.  That would be the sample
space if you were only going to toss the coin one time. Here is the 
sample space:   

{HHHH,HHHT,HHTH,HHTT,HTHH,HTHT,HTTH,HTTT,THHH,THHT,THTH,THTT,TTHH,TTHT,TTTH,TTTT}

I've colored red all the cases that have three heads and 1 tail.

That's 4 out of 16, so the probability is  which reduces to 

So th of the time on the average you'll get three heads and 1 tail.

Edwin