Exactly this. It can absolutely be a fulfilling relationship, but it takes significantly more work than being a pair does.

It also isn’t a good fit for everyone. You have to be not only extremely confident in the existing relationship, but also secure enough in yourself to understand that your partners are going to have independent relationships with each other just like you have an independent relationship with each of them. A lot of people just struggle with that.

Personally I find it to be a very rewarding experience, but I full expect that to not be the case for most.

If it is something OP decides to engage with I would definitely recommend lots of reading on poly relationships, seeing a relationship counselor who specializes in open \ poly relationships, and visiting poly forums to talk about the potential pitfalls \rewards.