NIC cages are not too hard to build. It does take some time and planing though, so have a few hours to do it.
You will need the grids, coroplast (or something for the base and to cover levels), zip ties, duct tape, utility knife, scissors and wooden dowels. Some people use wood for the base and levels, so you would need the wood, screws, glue, saw and it can be good to use stick on tiles to cover the wood.
The number of grids depends on how big you want the cage. You will probably need 3-4 sets (of 13-16 grids) to build a good size cage for 3 rabbits. Start looking now so you can find them on sale or even at garage sales or used online (craigslist etc). There are some brands that have grids with larger holes, these are not safe for rabbits so best avoided. The ones you want will have holes that are about 1.5" and there should be 9 holes along the grid (81 holes total per grid). Some have 8 holes across that can be good as well.
It can be better to get a 4X8ft sheet of coroplast. This means there should be minimal seams in the cage. You might be able to find some free or cheap stuff that is election signs other other signs. If there is print on it, you may want to get some stick on tiles as it can be more slippery and harder to clean than unpainted stuff.
You should get about 200+ zip ties. The 7.5" long ones are best (more for the strength than the length). While the grids come with connectors, they are a pain in the butt and zip ties are much easier.
Duct tape is for taping the coroplast. I like the Duck brand and it comes in fancy colours.
The dowel is to support and levels. You want it to be cut a couple inches longer than the level. It should be at least 1" thick, but not more than 1.5 as it won't fir through the bars.
Building it is pretty easy. Zip tie the grids together (at least 2-3 ties per joint). The cage should be at least 2 grids high, but if you are making it for 3 rabbits then 3 grids or taller would be good. Figure out how big you want it and make it that big. For the levels, do it one grid at a time as trying to zip tie the level before makes it much harder to put in the cage. Put the dowel under the level and zip tie it on. Cut the coroplast to the right size (if on the inside cut a bit smaller so it will fit, if on the outside cut a bit bigger). Make sure the add for the edges, 4-6" on each side is enough and tape those on. One thing I do is put some plastic poster hanger or something similar (I just found some stuff at a hardware store that is much easier to use and cheaper, not really sure what it is called though, but was near the wire mesh and screen doors I think) to cover the edge of the coroplast to prevent chewing. Also make coroplast to cover the levels.
