No, not in my opinion. A flemish would fit in that cage, but would not have room. Consider that the cage is 42 inches long/wide. You're going to need a litter box that's at least probably 18 inches long and 14 inches wide. Add to that a food bowl, water bowl, some toys and you don't have much room left. Laying down with legs stretched out, Flemmies can be as long as 32 inches. Sitting up, you need room for not just their body, but also their 8 inch long ears. Keep in mind they need room to hop along a little way as well.
In my opinion, a 3x3 cage is fine for a breed like a netherland dwarf, polish or mini lop. But I do like my pets to have as much space as feasible. If you're really tight for space, at least go to 4 grids long, though five would be my personal minimum for a flemmie. Bandit weighs 3kg (that's a third the size of the Flemish) and laying down