I know you care about your grandchildren. As you said, you wouldn't be here if you didn't. I was simply assuming you didn't know the dangers of used seats. Used 3-4 times a month, or used daily, you don't know the history and know that these seats are safe, and will protect your grandchildren. That's simply the end of it. Used carseats are an unknown.
And it didn't sound like you knew of extended rear facing, since you asked for forward facing seats for one year olds (the recommendation since 2002 has been to rear face to the maximum limits of the convertible seat).
http://carseatblog.com/5168/why-rear-facing-is-better-your-rf-link-guide/
It's quite obvious to me that you care, and that you care a great deal. But as a professional carseat technician, I'm simply telling you the safe thing to do, and will give you safe options. That's my job, and this is what I do. If you didn't want safe options, you wouldn't be here. Used forward facing seats for one year olds are not safe options.
I'd look at Cosco Sceneras for the two youngers. They rear face to 35 or 40 pounds, and usually run about $40. They're also fairly narrow. The Cosco Apt is wider, but about $55. You'd probably be able to find a Diono Radian between two Apts, but that's about it (rear or forward facing, and it doesn't matter which one, they're all the same size so find the best deal you can). Otherwise, I'd look at an Evenflo Maestro if the two year old will also be forward facing. It's not super narrow, but fairly narrow, and about $70. These are all brand new prices, and they're perfectly safe.
When the newest grandchild arrives, will you be using an infant seat, or the convertible you get? The Apt and Scenera both do fit newborns, so you'd only need to recline it more for the baby, then just plop them in. You will need to keep track of harness height, as the straps need to be at or below rear facing (and at or above forward facing), but not so low that they fall off of the older child's shoulders. So if you're going to go from newborn to 18 month old in this seat often, a seat that doesn't need a rethreading of the harness would be easiest. Unfortunately, none of them are narrow. The Evenflo Triumph 65 is the favorite amongst grandparents and nannies who have multiple kids, but it's very wide. However, you never need to deal with the harness height. The First Years True Fit doesn't have rethread harness, but it fits newborns and older kids with a quick adjustment. And it's wide. The Britax Marathon, Pavilion, Boulevard, and Advocate are all non-rethreading harnesses. But again, wide.
Are you going to be taking the seats out often and reinstalling them when you have the grandkids with you, or will you just be leaving the seats in all the time? Either way it's a good idea to meet with a certified technician so you are taught the proper way of installing them, especially if you're doing a three across situation which can be tricky (you need to make sure each carseat is tight as if no other carseats are in the back, so it often requires a specific order and playing around to get right). But if you're going to be reinstalling often, seats that are easier to install would mean that there would be less chance of error every time you install (that's nothing against you, there's a little room for error every time anyone installs a seat).
Wendy
