Thank you for wanting to help these kitties!
First of all - feral cats (or stray cats that have become scared of people and thus act feral) are like wild animals, and you have to think of them that way. They will be scared of you. They can come to trust you - and that's the difference between a feral cat as a wild animal and something like a possum - but building that trust takes time.
And believe me - most of us know EXACTLY what you are going through! Once you start helping one - it's like you've lit up a neon sign and cats in need show up.
There are several options:
1) Use the link Katie provided to find an organization near you that will take over. If you can't find a group near you in that link, try this one: http://www.pets911.com
2) Find a vet or organization or shelter where you can at least borrow or rent a trap. Trap the kitties yourself, get them spayed, neutered and ear-tipped (ask the vet to nip the top of the left ear off - this will indicate to anyone else that comes along to TNR (TNR = Trap, Neuter, Release) cats that these cats have already been spayed or neutered.
If you need low-cost spay/neuter services, you can use the same pets911 link or the link in my signature line to search for local low-cost services.
3) If you want to relocate the wild kitties rather than simply releasing them where they are so that you can feed them at your home (think of them as 3 outside pet kitties), you will need 3 large crates. The crates must be large enough so that they have a place to sleep, a place for food and water, and a place for a litterbox. To relocate ferals, they need to be in the cages for about 3 weeks. You would leave dry food in the crates for them to free feed on, but give them a strong smelling wet food at the exact same time every day. After 3 weeks, they'll be accustomed to the schedule, so that when you open the crate doors for them to leave - you simply keep putting the same wet food out the same time every day in the same place the crates were and hope for the best. 3 weeks seems to be the right amount of time to ensure that they'll keep coming back. You may be able to borrow large enough crates from a shelter.
4) Trap them, have them sterilized, release them where you trapped them, and keep stopping by to feed them.
5) Work on finding a farm or store or restaurant or some other local business that is willing to care for the cats but will keep them on their property as "mousers." In the meantime, trap them, have them sterilized & ear-tipped, and release them where you trapped them.
6) Trap them, have them sterilized & ear-tipped, release them where you trapped them, and stop coming by to feed them.
7) Do nothing. Stop feeding them.
These last 2 sound cruel, and kind of are. But if you cannot find an organization to help, and you do not have the time or resources to have them sterilized, you really ought to stop feeding them. It may sound cruel, but I think helping to keep them alive and healthy so they can procreate and create more homeless cats is even more cruel.
But if you can get them trapped and sterilized, but relocating them isn't an option, and you won't be able to regularly feed them, at the very least getting them spayed and neutered will help prevent more unwanted kitties - and if you can't keep up the feeding, it's best to let them find a "more reliable" source of food (even if it's garbage - at least they'll know when and where to find it as opposed to becoming dependent and then not having food for two weeks when you go on vacation or get sick or something).
Of course, keeping the one cat that was dependent on your grandma and having the other two sterilized and release them is an option.
As furryferals pointed out, if you decide to bring her in, you will need a separate room or a large cage (they make cat cages that have multiple levels with plastic shelves - it would be an ideal place for new kitty).
If you decide to bring her in, let us know so we can give you lots of pointers on socializing the new addition to your family.