Correct position of adjective?

Solution 1:

The third sentence is

The police seem certain they will find the people who is responsible for the attack.

"People" is a plural word so it requires "are" instead of "is", and the present tense is incorrect too.

The second sentence is

The police seem certain they will find the responsible people for the attack.

The adjective is placed before "people" and the meaning of "responsible people" is that they are "well-behaved and law-abiding", which in the usage is clearly not so.

The first sentence is

The police seem certain they will find the people responsible for the attack.

and this is the correct sentence, as can be seen by adding some implied words:

The police seem certain they will find the people [who were] responsible for the attack.