Description of $GL_3$-orbits on the quadruple projective varieties

This article gives a description of the diagonal $GL_3$-orbits on the quadruple projective variety $(\mathbb P^2)^4$. We give explicit representatives of orbits, and describe the closure relations of orbits. A distinguished feature of our setting is that it is the simplest case where $\mathrm{diag}(GL_n)$ has infinitely many orbits but has an open orbit in the multiple projective space $(\mathbb P^{n-1})^m$.