Actually, if each one of your two external drives can handle backing up all three computers at once, you don't really need anything else nor to spend another penny. Just manually back up all three computers to the first drive, then repeat the process to the second drive. Do not connect either drive to the computers except when updating the backups. Again, even though it is less convenient, you do not want to use any method that will require that you backup drives are connected to the computers or even to power except while updating a backup.
Store one of the drives at home where you can get to it easily to frequently update the backups and store the other drive offsite somewhere, such as at a trusted neighbor's, friend's, or relative's house or, if you already have one and it's large enough, a safe deposit box at your bank.
Just swap the two drives once a month, or more often if possible, so the offsite drive is as up to date as possible. The more often you swap the drives, the less data you will lose if you should ever have to depend on the offsite backup.
I'm also on Social Security (fortunately, it is supplemented somewhat by a pension) so I understand being on a budget. Still, you may want to look into a cloud backup plan like I mentioned earlier. I use Carbonite on just my desktop computer since the data I keep on my notebook computers is also on the desktop computer. If I add or change data to one of notebooks while, say, on a trip (normally, the only time I use my notebooks), I backup the added or changed data on a thumb drive that I keep in my pocket or my purse until I can get home and transfer the new or changed data to my desktop. Carbonite is costing me only $5 as month ($60/yearly) for the one computer and ensures that, should I lose both my computer and onsite backups, I can recover any data that was added or changed since the last time I updated my offsite backups.
In your case, if one of your computers has room to also store the data on the other two computers, then you would need to backup only that one computer to the cloud instead of all three, same as I do. Most people have at least one vice they can cut back on to be able to afford $5/month. It all depends on how valuable your data is to you.
An added advantage to a cloud backup is you can access your data from any computer anywhere. That's why I can get away with keeping so little data on my notebooks. If I should need data that I didn't put on the notebook while I'm on the road, I just retrieve a copy from Carbonite.
