Age ratio problem

Let Tom's age be $t$. Dick's age $D = 3t$. Harry's age $H = 2D = 6t$.

Average age of all three = $\frac{t + 3t + 6t}{3} = \frac{10t}{3}$. Given: $D = \frac{10t}{3} - 1$ $3t = \frac{10t}{3} - 1 \implies \frac{t}{3} = 1 \implies t = 3$.

Harry's age $H = 6t = 18$ years.