Closed triads will not spare you from having to unlearn toxic monogamy. I think there is a misconception, especially amongst newcomers to polyamory, that as long as their partners were only dating people they were also dating, they’ll never be jealous. After all, they can’t be scared that their metamour is going to steal their partner away if their metamour also has a romantic reason to stay. Toxic monogamy says you must constantly be defending your partner from the attempts of the world to steal them from you. As long as the only person with romantic or sexual access to your parnter is also your parnter, you can have several partners without having to unpack that defensive mindset, you can just switch to defending them as a pair.
But it doesn’t work like that. If you don’t unpack toxic monogamy, even your closed relationships won’t survive. Toxic monogamy won’t spare you because your relationship is closed. You’ll feel jealous when they’re having alone time together, you’ll feel uncomfortable when they kiss each other if they don’t then immediately go over and kiss you, toxic monogamy will whisper in your brain late at night that your partners love each other more than they love you, strangers will tell you that your partners are one day going to leave you to be monogamous and if you haven’t unpacked why that’s bullshit, you’re going to believe it, even if only a little. There is no way of being polyamorous where your own toxic monogamy will declare you one of the good ones. You must unpack it now or when you are finally in that relationship you’ve always longed for, you’ll fuck it up.