Behavior in Older Cat with 2 new cats

I have a12 yr old female cat that just lost her playmate of 13 yrs. I got 2 female cats about 7 months old thinking that they would keep her company. I kept them apart for about a week, then let them in the house together. She hisses at them and stays away. They go everywhere in the house so she doesn't have a place to hide that they can't go. 6 weeks later this is still going on and they like her but she doesn't like them. Well 3 days ago she started doing # 1 and #2 on my kitchen rug. I moved her litter box in a room by itself and she was going there. Now on my rug. I think one of the cats have gone in hers because when I took her to it she hisses and runs away. She has gone on my throw rug now 3 days in a row. How can I change this. I cleaned her litter box out and put it back in the room. How do I keep the other cats from using it.
I don't know what to do. Is she getting me back for bring the other cats in. She used her box for the first 6 wks I had the cats, and during that time I think the cats used it too. Should I put her in a room by herself until this stops or or put the other cats back in a room by theirself?

Any new cat in a house will bring out the hisses and swats in a resident cat. A female will bring out worse problems if the resident is female too, and two new females is throwing fuel on the fire! She may settle down with them over time, but so soon after losing her friend may be too soon, so give it some time, and maybe keep them separated for a few weeks.

Females (spayed or not) tend to be more territorial then males, especially when you bring in a rival female(s). It might have been more accepting to bring in a younger male that was neutered. But all is not lost as I don't know your reasons for choosing females.

Anyway, get a few more litter boxes. She probably resents sharing her litter box with the new females.

And you can sprinkle all with cornstarch baby powder so they smell the same or use some vanilla extract on their nose/chin and tail.

But start by separating them and re-introducing them slowly.