Looking at the photo, that's already a pretty substantial sword for a 3-gallon tank. It can definitely survive there, especially if you like the overgrown jungle look, but I wouldn't expect it to stay "small" for long.
Amazon swords are heavy root feeders, so root tabs will likely help if the substrate is mostly inert gravel. The plant may do fine for a while on the nutrients already available in the tank, but long term it will probably appreciate additional feeding.
That said, if your goal is a lush, overgrown betta tank and you don't mind the sword becoming the dominant feature, there's nothing wrong with letting it grow and seeing where it takes the scape