Like Yayi said, just give her time, and be sure her necessities are nearby in case she's too scared to leave her hiding place to use them.
Let her come out in her own time.
A couple of ideas...(taken from a reply I did to another post...hope you don't mind)
For the couple weeks before, bring some things that have Ball's scent on them (perhaps things he's laid on, like clothes, towels, etc.) into Scrounge & Dina's home, bringing in new things every couple days to make sure the scent still exists. This will get them familiar with Ball's scent, and they will feel less intimidated going into his territory this way. Also do the reverse of bringing stuff with their scent to Ball.
I would normally suggest separating them for about a week (new kitty in a room with kitty necessities, with old kitty having the run of the house), but because you're bringing two kitties into a one-kitty household, this may not work as well. You could do this, but shorten it to a day or two, and introduce them to see how they relate to each other. If you do the scent thing I mentioned above, they shouldn't react so severly to each other as a normal introduction would bring about. If you find that one of them is reacting strongly, separate them again for a day. They'll get the idea.
As I put into posts about such things, there is something you should be aware of. If you introduce the three of them, and things are going just fine, be warned that they will be fine for about a week, and might be not-so-great for about two after that. The thing is that if things go great right away, the kitties may be thinking it's just a temporary situation, and once they realize it's not, they may have to be separated for a time until they get along.
Other than those things, I would just be sure they have their own places to go if they wish to avoid each other (they being separated into two groups, Scrounge & Dina vs. Ball). I would make sure until you're certain they're fine with each other that their litterboxes, food, water, and comfort items (such as beds, etc.) are in separate spaces. Don't worry if one kitty decides to hide for a week or so.
All in all, it sounds like it'll go just fine. They'll work out their differences, and all will be a happy family.
The most important thing to remember is to take it all in their time
Feel free to ask any more questions!!