Three Heads Or Bust

You flip a fair coin repeatedly and stop as soon as one of two patterns has appeared in consecutive flips: HHH or THH.

The race, written as a chain whose state is the useful suffix of what you have seen so far:

state on H on T
start H T
H HH T
HH HHH wins T
T TH T
TH THH wins T

What is the probability that THH appears first?

Answer format: round to 3 decimal places.