Given:
- Three coins are tossed together.
- We need to find the probability that exactly one coin shows a head.
Step 1: Find total number of possible outcomes
Each coin has 2 possible outcomes: Head (H) or Tail (T).
Total outcomes for 3 coins:
\[
2 \times 2 \times 2 = 8
\]
These outcomes are: HHH, HHT, HTH, THH, HTT, THT, TTH, TTT.
Step 2: Identify favorable outcomes (exactly one head)
Favorable outcomes are those with exactly one head:
- HTT
- THT
- TTH
Number of favorable outcomes = 3.
Step 3: Calculate probability
\[
P(\text{exactly one head}) = \frac{\text{Number of favorable outcomes}}{\text{Total outcomes}} = \frac{3}{8}
\]
Final Answer:
\[
\boxed{\frac{3}{8}}
\]