Poisson Probability Density Function (with mean $\lambda$) =$\dfrac{ \lambda^{k}} { \left(e^{\lambda}k!\right)}$,
We have to sum the probability density function for $k = 0,1$ and $2$ and $\lambda = 3 $(thus finding the cumulative mass function)
=$\left(\dfrac{1}{e^{3}}\right) + \left(\dfrac{3}{e^{3}}\right) + \left(\dfrac{9}{2e^{3}}\right)$
=$\dfrac{17}{\left(2e^{3}\right)}$