Finding the Pareto optimal set with preferences for 3 individuals
I am somewhat unsure where your exact difficulty lies (and hence what your question is). As you write the two Pareto impromevents are $(a,b),(a,c)$. Meaning $b$ and $c$ can be improved upon and are not efficient. Also $(a,d)$ is an improvement, notice that individual 2 is indifferent while others prefer $a$ to $d$. Alternative $a$ cannot be improved upon, and hence it is Pareto-efficient. So the Pareto-set would be $\left\{a\right\}$.