The denominator isn't consecutive

Algebra Level 5

$\large \dfrac{3}{1\times 2 \times 4} + \dfrac{4}{2 \times 3 \times 5} + \dfrac{5}{3 \times 4 \times 6} + \ldots$

If the above summation can be expressed as $\dfrac{a}{b}$ where $a,b$ are co-prime natural numbers, find $a+b$ .