FF3 nes had you fighting few enemies as well, however, the issue here is mostly speed. Also, FFIII nes has you fighting more enemies, more regularly, like 5 enemies at once sometimes. Instead of upping that number, like the snes FF's did to like, 5-9 enemies at once on average, it's almost always 1-3 enemies at once, which is 3d FF standards.
Factor in slow battle speed, ugly graphics, enemies with hp sinks because there are LESS enemies now so they try to make up for that, and you got a bad remake.