Aggressive Kitty

I have two cats and a dog.  I feel really bad because Adele was my first kitty.  My husband and daughter got me Colby for Christmas and Adele was hesitant but dealt with Colby.  Then our wonderful, elderly dog passed away and we got a new dog.

Now, Adele hides on top of the fridge and has stopped using the litter box because Colby harasses her when she comes down and Allie (new dog) thinks that since Colby likes to play, so will Adele.

I want my pets to live relatively peacefully together.  I love each of them and can't stand the thought of getting rid of one but Adele is so unhappy and it would be so selfish to make her remain in such an unpleasant environment for her.

Long story short, does anyone reccomend I ask my vet about something to chill Colby out, or maybe even give Adele an anti-anxiety med? I've seen some folks say his ir her vets prescribe Xanax... .25mg.  That will knock out a full grown man if he's never had it before so I'm slightly apprehensive.

I appreciate ANY help!!

Did you do an proper introduction? Here is how to introduce cats to dogs (http://www.thecatsite.com/a/introducing-cats-to-dogs) and cats to other cats (http://www.thecatsite.com/a/introducing-cats-to-cats). You should start the introduction process over again and separate them. Put Adele in a separate room with a litter box and food and follow the advice found in the articles of slowly introducing her to the other cat and dog and mix their smells. Some things to help the introduction process is feliway diffusers and spirit essences do work well to calm cats (http://jacksongalaxy.com/spirit-essences/). 

After you slowly introduce them make sure you give Adele the deference she deserves as the elder cat and never allow either the dog or the other cat to go after her. No wonder she is not using the litterbox as she feels constantly under attack. Make sure to train the dog to not go after the cat and teach the command "leave it" or "down".  I am not sure why you would think about rehoming her because she is clearly the victim here and it is almost impossible to find a good home for an older cat and she would most likely end up being killed in a shelter if you abandon her. There are a lot of people that have multiple cats and dogs and they are able to live harmoniously after proper introductions and some training. I have two cats and a dog too and I have to really watch out not to let the kitten bug my other cat and I get her interested in a toy when she tries to attack Frank. My dog is super old now but when she was younger and we first got cats I trained her to listen to my sit command if she tried to go after the cats.