Hi there, sorry to hear of your losses!
I would separate the remaining two girls, even if they have been with each other for a while now, aggression can come out of the blue sometimes so best to just separate them until the time comes they can be reintroduced.
If you can, I would get two more girls to round it back to 5, 3 isn't enough to spread aggression around and you could end up with some more dead girls and that wouldn't be fun! With that done, I wouldn't get any snails they poop way too much and don't actually really clean except for algae removal. But they poop more than their worth so you'd be siphoning out even more poop is what I'm saying.
What kind of algae eater do you have in there? You might be over stocked depending on what it is.