You'll need to do a little research on the amount of hard drive space you're going to need for your specific application. Say you want the cameras to record 24 hours a day and have footage saved for 30 days before it overwrites the oldest existing data, then you'll need x TB of hard drive space to accommodate that. Some systems have motion activation and only record when they detect motion, so it will save some hard drive space if there isn't a lot of activity. Then you'll need to consider whether your hard drive array is mirrored so that when a hard drive fails (and they will fail over time so plan on it), the data will exist on the rest of the array while you replace the drive. If your saved footage isn't important then maybe having a redundant array isn't important, but without it the saved data is lost. A nice feature to have is hot swappable hard drives so you can replace a hard drive and rebuild the array without having to turn the power to the unit off. You just unplug the failed drive, replace it, and hit rebuild array. You want to keep at least one hard drive on hand at all times to replace a failed drive. Also over time you may lose a camera and need to replace it. It can be hard to find obsolete model cameras 5 years down the road even from mainstream manufacturers like Samsung. If you need 3 cameras and the kit comes with 4, consider the 4th a backup for failure. Other than that just plan on factoring in installation for the 3 or 4 cameras. They may run on PoE and need ethernet cables run to each one and a PoE switch to power them, or they may be wireless and just need a power outlet near each one. I've only ever worked with cabled IP cameras and not the wireless.
Edit: I suppose this goes without saying, but if your cameras are going to be located outdoors, get outdoor cameras
.