There's nothing that proves that Jade is together with Dave and Karkat aside from heteronormativity. In fact the house next to Karkat's is Jade's green, and not olive or lime blood colored. Not only that, but it completely goes against Dave and Karkat's arcs. Also, Davepeta is right there.

Jade may have two hands, but if her Jadebot dreaming up extra arms is anything to go by, I would say she could most certainly be with Karkat, Dave AND Davepeta as well.

I mean, is a Poly Relationship with the three just being happy together, perhaps the four of them, perhaps more, ‘heteronormativity’? Genuine question. Jade being with them doesn’t invalidate the two of them being together, and Dave and Karkat are most definitely Bi/Pan.