This problem takes a bit of work. There are six cases to calculate then add them up.

You can have anywhere from zero to five heads. You cannot have six. Why.

In the case of having three heads and seven tails, the tails separate the heads.

So there are eight places to place the three heads: _T_T_T_T_T_T_T_.

That means that there are ways to do that.

So the probability of this case is

Now you have five more cases to work out.