Automorphisms of free centre-by-metabelian groups of small rank

For a positive integer n, with n >= 2, let F-n be the free group of rank n and let C-n = F-n/(F-n '', F-n), that is, C-n is a free centre-by-metabelian group of rank n. Write Aut(C-n) for the automorphism group of C-n and T-n for the group of tame automorphisms of C-n. It has been proved by E. Stohr (Arch Math 48:376-380, 1987) that for 2 <= n <= 3, Aut(C-n) is not finitely generated and for n >= 4, Aut(C-n) is generated by T-n and one more automorphism of C-n. For n = 2, we find an infinite minimal subset Y of Aut(C-2) such that Aut(C-2) is generated by T-2 and Y. For n = 3, we find a subgroup of Aut(C3), generated by T3 and two more automorphisms of C-3, which is dense in Aut(C-3) with respect to the formal power series topology.