I recently met a nice couple looking to add someone in their Relationship, they say it's not only for sex . . .
I can't understand if the whole unicorn thing is just for couple looking for sex or if it's for any couple looking for a third person.
What are YOU looking for? Start by asking yourself that. Then figure out YOUR personal boundaries around what YOU will or will not accept in a relationship, and ask them if their boundaries match up with yours.
For example, do you want to be free to date other people outside of this couple? If so, great - let them know that. If they frown on the idea and tell you they only want a woman to be faithful to them from the very start, you won't be very compatible or happy in this situation.
Another example: Do you want to be able to have "alone time" and dates with each of them separately, and to build your relationships with each of them at a natural pace? If so, let them know. If they balk at that and tell you that they only want someone who will have sex with them both together (threesomes only) and you wouldn't be "allowed" to have private one-on-one time with either of them, it's doubtful you will feel like anything but a fucktoy for them.
Do you want to be able to contact one or the other of them whenever and however you want and to have privacy in your communications? Great, not unreasonable at all - let them know. If they tell you that all contact must be made through one of them only, to be relayed and shared with the other, and that you will not be allowed to contact either of them any other way, that will be problematic.
Poly relationships are no different from monogamous relationships in many ways. Essentially, in ANY relationship, people want to feel respected as individuals, to feel they are a valued person in others' lives, and that they are heard when they need to express themselves. It's also imperative that you feel safe to be yourself and free to make your own choices.
If they only want things on their terms and won't care about your boundaries, which is typical of most unicorn hunters (called that because they have ideals that are impossible to fulfill), then be smart and head in the other direction. Remember that you count, you matter, and just because a couple claim to be poly doesn't mean they know how to do it well. They might be more inclined to a swinging mindset where it is focused on recreational sex without emotional involvement. If that's what you want, that's fine, too. So, like I said above, the place to start is to ask yourself what YOU want -- and then to care about yourself and value yourself so that you won't compromise on what's important to you.