A fair coin has 2 sides.
Flipping three fair coins means a total of 2^3 = 8 outcomes.
We are interested in getting two tails and a head which can happen in 3 ways.
{HTT, THT, TTH}
Odds in favor means favorable outcomes : unfavorable outcomes
Odds against means unfavorable outcomes : favorable outcomes
For the given case, we have
Favorable outcomes = 3
Unfavorable outcomes = 8 - 3 = 5
The odds against of getting two tails and a head are 5:3