Step 1: Idea
Solubility rises in direct proportion with pressure.
Step 2: Scale from 1 atm
At 1 atm the solubility is $0.03$ M. At 3 atm it is three times this.
Step 3: Result
$3 \times 0.03 = 0.09$ M.
Step 4: Cross-check
Dividing back, $0.09/3 = 0.03$ M atm$^{-1}$ equals the given Henry constant, so the value is consistent.
Step 5: Units and checking
The Henry constant has units of mol per cubic decimetre per atmosphere, so multiplying by pressure in atmosphere leaves mol per cubic decimetre, which is molarity (M). The product $0.03 \times 3 = 0.09$ M therefore needs no further conversion.
Final Answer:
The solubility is 0.09 M. This is option (C).
\[ \boxed{\text{(C) }0.09\ \text{M}} \]