Writing function using sigma?
Formally this can be easily done using a truth table. But if you feel lazy to fill 16 rows, you can do it in your head in a bit shorter way.
Let's take a look at the original expression:
$$g(w,x,y,z)=(x+ y ̅+ z ̅  )( w+ y ̅+ z ̅  )( w+ x ̅+ y ̅  )$$
Since it is POS, it will be more natural to work in the \$\Pi\$ notation and &quot;inverted&quot; logic.
So g will have the value of 0 whenever either of the three maxterms is zero. So let's start with first term:
$$(x+ y ̅+ z ̅  )$$
When is it taking the value zero? It is when \$(xyz)=(011)\$ and the value of \$w\$ is &quot;don't care&quot;. That is it is corresponding to the rows \$0011\$ and \$1011\$. That is the rows number \$3\$ and \$11\$. So these are going into the \$\Pi\$ representation.
Then you repeat the same for the rest of the terms and complete the \$\Pi\$ representation.
The \$\Sigma\$ representation is simply the complementary one (that is you fill the numbers out of the 0-15 range which do not appear under \$\Pi\$
P.S. If you wish you can add more formality to this method and first write down the explicit expansion of each maxterm into a canonical form as you did in the question (plus reducing the identical terms). Then you will have the exact same analysis as above, but for more terms without &quot;don't cares&quot;. But I would say it is just extra work without extra benefit.