Sorry to hear you are experiencing that! I have like today realized that a lot of people seeking their mythical unicorn are motivated by the sex and not the relationship possibilities, and I'm like 'woot? threesomes are SOO over-rated!'. Besides, arranging a threesome isn't even that hard if you know the right people. Having a relationship is sometimes hard, but worth so much more than the occasional kicks you get out of group sex with random people.
The advice you have already received is really good. Don't build huge expectations about meeting the 'perfect couple' - date individuals instead for a while, and look for couples that already have some experience in poly (the local community is a good place to start). If you meet potentials, bring it up from the get go that you want to date and get to know them both individually before the sex, and make a commitment to yourself NOT to agree to anything you are not comfortable with. Triads can also happen when three single people get together, so don't limit your line of vision to established couples only.
Although group dates are fun, I prefer to get to know people and to have sex one-on-one. I don't know if you could benefit from seeking out more 'family-minded' polyfolks. They tend to have a bigger emphasis on seeking a 'good fit' to their family, and don't want to bring random people in to their children's life, but really prefer to get to know the person first.
Me: bi female in my twenties