>>We did a lot of these assignments and no one assumed that they will be hired if they complete it. Its about how you communicate your intent.
Be upfront that finishing the assignment doesn't guarantee a hire and very likely the very people you want to hire won't show up.
Please note that as much as you want good people to participate in your processes. Most good talent doesn't like to waste its time and effort. How would you feel if someone wasted your time and effort?
I am in germany, so by far not the same situation as in other areas of the world. If I would get such an assignment myself and I have the feeling that this will help the company and also me to verify, if it is a fit, I will do that 1 to 3 hour task very happily.