My silly relationship math logic from a male POV is this:

The energy required communication wise is the sum of the differences of all people in a relationship and the number of participants.

So how does that work out?

Let's pretend Melissa is 30% different then me, this means to make the relationship successful, it requires 230% energy output (preferably split equally) to manage it. Over time if true openness happens either person can manager this and things can flow.

Now lets add a third partner "Joe or Jane" to the mix. This person is 10% different then Melissa but 40% then me.

There is the 300% from the base, and then 10% + 30% + 40% or 80%, for which any partner has to manage it (380%)

Workload goes up as does the reward.