Although technically you could back up the computers to each other, that isn't a normally recommended procedure. An external hard drive is normally used. I would recommend 2 external hard drives so each computer will have two backups, which is safer than having just one since hard drives can fail or backups can become corrupted. When set up, the computers will alternate between the 2 external hard drives. You can back up all 3 computers to one hard drive.
The hard drives need to be 2 - 4 times larger than the total amount of data, from all 3 computers, you are backing up.
The initial backup can take a long time, you just have to be patient. I would do the initial backup one at a time so you don't have 3 computers trying to write to the same drive at the same time.