On the torsion-free nilpotent fundamental groups of smooth quasi-projective varieties of rank up to seven

Let $X$ be a smooth quasi-projective variety. Assume that the (topological) fundamental group $π_1(X, x)$ is torsion-free nilpotent. We show that if the first Betti number $b_1(X) \le 3$, then $π_1(X, x)$ is isomorphic to either $\mathbb{Z}^n$ for $n = 1, 2, 3$, a lattice in the Heisenberg group $H_