If you're thinking of getting involved with a couple, I'd seriously look and see if they have any practical experience dating in this dynamic. The internet is full of couples looking for that third to "complete" their relationship or because they just want to "share the amazing thing they have". Sadly, most of these couples have no idea what it means to date as a triad and are often looking for that third to fill a very specific role. Often there is an expectation that you will love both of them equally and if you end up falling in love with one of them a bit more than the other, all sorts of nasty ensues.
And yeah, the fact that they seem to think that you can prove that there will be no jealousy issues seems to suggest that they really don't understand jealousy or understand themselves enough to deal with jealousy when it pops up.
I don't know if this specific couple fits this dynamic, but the fact that they are looking for this over craigslist just doesn't bode well. I'd run the other way if I were you.