I have a 1x1x1 POSCAR, I need o make a 3 square root 3X1X1 supercell. I understand VESTA cannot make it, are there online tools available for generating this? I have attached my POSCAR
you could also try a command line program atomsk. The code can work with different file formats and performs various transformations of atomic systems, like creating supercells, rotation, deformation, inserting dislocations etc.
http://atomsk.univ-lille1.fr/
Pierre Hirel, Comput. Phys. Comm. 197 (2015) 212-219. doi:10.1016/j.cpc.2015.07.012